[PATCH v3 0/4] KVM: arm64: Fix HCR_EL2 for non-protected VMs in pKVM

[PATCH v3 0/4] KVM: arm64: Fix HCR_EL2 for non-protected VMs in pKVM

[Fuad Tabba]

Hi folks,

Changes since v2 [1]:
- Patch 1: apply the FGUs on a CPU without FEAT_FGT instead of moving
  the check in handle_tlbi_el1() (Oliver [2]). Wei-Lin's Reviewed-by
  dropped with the rewrite.

In pKVM, EL2 sets a non-protected VM's HCR_EL2 in pkvm_vcpu_reset_hcr(),
which misses the RW, TID5 and TTLBOS handling of vcpu_set_hcr(), and
takes only TWI, TWE and VSE from the host. As a result, an AArch32 VM
can't run, a VM can read GMID_EL1 or execute a TLBI OS its ID registers
hide, and the host's TVM, VI and VF never reach it.

The second patch clears RW for an AArch32 vCPU. The third also takes
from the host, for a non-protected VM, the bits the host varies with the
VM's configuration or at runtime: VI, VF, TVM, TID2, TID4, TID5 and
TTLBOS. The rest stay EL2's, and a protected VM still takes only TWI,
TWE and VSE.

The third patch depends on the first: once TTLBOS reaches the VM, a
trapped TLBI OS from a non-nested guest on a CPU without FEAT_FGT hits
a WARN in handle_tlbi_el1(), as it does without pKVM. The first makes
the FGUs apply on such a CPU too, so the access is UNDEFINED before it
reaches handle_tlbi_el1().

The last patch adds a selftest that checks a feature hidden in an ID
register is UNDEFINED in the guest. Its TLBI OS case fails in pKVM
before the third patch.

VSE still comes from the host as before. Syncing it back after delivery
is a separate fix [3].

Based on Linux 7.3-rc4 (93f51579e7df2).

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260928064643.3265087-1-fuad.tabba@linux.dev/
[2] https://lore.kernel.org/all/arqeRSIoEwurSrya@kernel.org/
[3] https://lore.kernel.org/all/20260921101030.1231605-1-fuad.tabba@linux.dev/

Fuad Tabba (4):
  KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT
  KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs
  KVM: arm64: Use the host's HCR_EL2 for non-protected VMs in pKVM
  KVM: arm64: selftests: Check a feature hidden in an ID register is
    UNDEF

 arch/arm64/kvm/emulate-nested.c               |   3 -
 arch/arm64/kvm/hyp/include/nvhe/pkvm.h        |   9 +
 arch/arm64/kvm/hyp/nvhe/hyp-main.c            |   7 +-
 arch/arm64/kvm/hyp/nvhe/pkvm.c                |  25 ++-
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/arm64/hidden_features.c     | 184 ++++++++++++++++++
 6 files changed, 214 insertions(+), 15 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/arm64/hidden_features.c


base-commit: 93f51579e7df248780214094418f205253383cc5
-- 
2.39.5

[PATCH v3 1/4] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

[Fuad Tabba]

triage_sysreg_trap() applies the FGU bits through the encoding's FGT
entry in the trap xarray, and populate_nv_trap_config() only adds those
entries on a CPU with FEAT_FGT. Without it, a hidden feature that still
traps reaches its handler instead: a TLBI OS trapped through
HCR_EL2.TTLBOS lands in handle_tlbi_el1(), which expects an EL1 TLBI
only from vEL2 and WARNs before the guest gets its UNDEF. A VMM can
trigger the WARN by hiding TLBI OS and having the guest execute one.

kvm_calculate_traps() computes the FGU bits with or without FEAT_FGT,
and the entries map an encoding to its FGT bit whatever the CPU
implements. Add them unconditionally, so that a hidden feature that
traps is UNDEFINED on any CPU.

Fixes: f5a5a406b4b8b ("KVM: arm64: Propagate and handle Fine-Grained UNDEF bits")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---

Notes:
    For a nested guest on a CPU without FGT, check_fgt_bit() now runs too,
    but it forwards nothing: a guest without FEAT_FGT has its FGT registers
    RES0.
    
    f5a5a406b4b8b added the FGU check to triage_sysreg_trap() behind an
    ARM64_HAS_FGT gate that populate_nv_trap_config() already had, so the
    check never fired without FGT.

 arch/arm64/kvm/emulate-nested.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 625604019fb32..b8529532b6e78 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2386,9 +2386,6 @@ int __init populate_nv_trap_config(void)
 				print_nv_trap_error(fgt, "FGT bit is reserved", ret);
 			}
 
-			if (!cpus_have_final_cap(ARM64_HAS_FGT))
-				continue;
-
 			prev = xa_store(&sr_forward_xa, enc,
 					xa_mk_value(tc.val), GFP_KERNEL);
 
-- 
2.39.5

Re: [PATCH v3 1/4] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

[Wei-Lin Chang]

On Tue, Sep 29, 2026 at 10:00:28AM +0100, Fuad Tabba wrote:
> triage_sysreg_trap() applies the FGU bits through the encoding's FGT
> entry in the trap xarray, and populate_nv_trap_config() only adds those
> entries on a CPU with FEAT_FGT. Without it, a hidden feature that still
> traps reaches its handler instead: a TLBI OS trapped through
> HCR_EL2.TTLBOS lands in handle_tlbi_el1(), which expects an EL1 TLBI
> only from vEL2 and WARNs before the guest gets its UNDEF. A VMM can
> trigger the WARN by hiding TLBI OS and having the guest execute one.
> 
> kvm_calculate_traps() computes the FGU bits with or without FEAT_FGT,
> and the entries map an encoding to its FGT bit whatever the CPU
> implements. Add them unconditionally, so that a hidden feature that
> traps is UNDEFINED on any CPU.

This reads a bit mechanical to me, can I suggest:

```
FGUs don't actually require hardware support, so don't gate fgt trap
config insertion on FGT. This is the only action required to allow FGUs
always, as kvm_calculate_traps() already computes the FGU bits with or
without FEAT_FGT.

This also fixes a spurious WARN when a TLBI OS is run on a guest in vEL1
without FEAT_TLBIOS on non FEAT_FGT hardware. In this configuration the
course-grained HCR_EL2.TTLBOS trap reaches the handler handle_tlbi_el1(),
which expects an EL1 TLBI only from vEL2. With FGUs enabled the UNDEF
will be injected without needing the specific handler.
```

Other than that, this is very nice! Sorry for missing this. It's way
better to have FGU regardless of FGT and handle unexposed TLBIOS with
FGU always. Thanks Oliver for suggesting this instead, and thanks Fuad
for digging in.

Reviewed-by: Wei-Lin Chang <weilin.chang@arm.com>

Thanks,
Wei-Lin Chang

> 
> Fixes: f5a5a406b4b8b ("KVM: arm64: Propagate and handle Fine-Grained UNDEF bits")
> Cc: stable@vger.kernel.org
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

> ---
> 
> Notes:
>     For a nested guest on a CPU without FGT, check_fgt_bit() now runs too,
>     but it forwards nothing: a guest without FEAT_FGT has its FGT registers
>     RES0.
>     
>     f5a5a406b4b8b added the FGU check to triage_sysreg_trap() behind an
>     ARM64_HAS_FGT gate that populate_nv_trap_config() already had, so the
>     check never fired without FGT.
> 
>  arch/arm64/kvm/emulate-nested.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
> index 625604019fb32..b8529532b6e78 100644
> --- a/arch/arm64/kvm/emulate-nested.c
> +++ b/arch/arm64/kvm/emulate-nested.c
> @@ -2386,9 +2386,6 @@ int __init populate_nv_trap_config(void)
>  				print_nv_trap_error(fgt, "FGT bit is reserved", ret);
>  			}
>  
> -			if (!cpus_have_final_cap(ARM64_HAS_FGT))
> -				continue;
> -
>  			prev = xa_store(&sr_forward_xa, enc,
>  					xa_mk_value(tc.val), GFP_KERNEL);
>  
> -- 
> 2.39.5
> 

Re: [PATCH v3 1/4] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT

[Fuad Tabba]

On Tue, 29 Sept 2026 at 15:54, Wei-Lin Chang <weilin.chang@arm.com> wrote:
[...]
> This reads a bit mechanical to me, can I suggest:
>
> ```
> FGUs don't actually require hardware support, so don't gate fgt trap
> config insertion on FGT. This is the only action required to allow FGUs
> always, as kvm_calculate_traps() already computes the FGU bits with or
> without FEAT_FGT.
>
> This also fixes a spurious WARN when a TLBI OS is run on a guest in vEL1
> without FEAT_TLBIOS on non FEAT_FGT hardware. In this configuration the
> course-grained HCR_EL2.TTLBOS trap reaches the handler handle_tlbi_el1(),
> which expects an EL1 TLBI only from vEL2. With FGUs enabled the UNDEF
> will be injected without needing the specific handler.
> ```

I cannot write commit messages! :) Thanks for the suggestion and the
review. I'll reword if I respin.

Cheers,
/fuad

[PATCH v3 2/4] KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs

[Fuad Tabba]

pKVM keeps its own copy of each vCPU's HCR_EL2 at EL2, initialised with
RW set unconditionally. Nothing stops userspace from creating a
non-protected AArch32 VM, and with RW set, entering one of its vCPUs is
an illegal exception return: KVM_RUN fails with KVM_EXIT_FAIL_ENTRY.

Clear RW for a vCPU that is AArch32 at EL1, when the CPU has AArch32
EL1. On a CPU without it, RW stays set and the entry still fails, rather
than EL2 switching *32_EL2 registers that are UNDEFINED there. The host
already rejects such a vCPU, but EL2 takes the vCPU's features from the
host and doesn't rely on that.

Fixes: b56680de9c648 ("KVM: arm64: Initialize trap register values in hyp in pKVM")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/pkvm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 459bd9eb7e4bc..62d432144d44c 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -54,6 +54,15 @@ static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 	else
 		vcpu->arch.hcr_el2 |= HCR_TID2;
 
+	/*
+	 * Without AArch32 EL1, leave RW set and let the entry fail with an
+	 * illegal exception return: the *32_EL2 registers EL2 would otherwise
+	 * switch are UNDEFINED there.
+	 */
+	if (vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT) &&
+	    cpus_have_final_cap(ARM64_HAS_32BIT_EL1))
+		vcpu->arch.hcr_el2 &= ~HCR_EL2_RW;
+
 	if (vcpu_has_ptrauth(vcpu))
 		vcpu->arch.hcr_el2 |= (HCR_API | HCR_APK);
 
-- 
2.39.5

[PATCH v3 3/4] KVM: arm64: Use the host's HCR_EL2 for non-protected VMs in pKVM

[Fuad Tabba]

For each vCPU, pKVM's EL2 builds its own HCR_EL2 and takes only TWI, TWE
and VSE from the host's value on each entry. For a non-protected VM it
has fallen behind the host's: on a CPU with MTE the guest can read
GMID_EL1, on one with FEAT_EVT2 but no FGT it can execute a TLBI OS its
ID registers hide, an interrupt injected without a vGIC (VI, VF) never
arrives, and set/way emulation loses the TVM trap it relies on.

For a non-protected VM, also take the bits the host varies with the VM's
configuration or at runtime: VI, VF, TVM, TID2, TID4, TID5 and TTLBOS.
They only pend interrupts for, or add traps to, a VM the host controls.
The traps exit to the host, which handles them as it does without pKVM.
The bits that change what EL2 does on entry and exit (E2H, RW, API/APK)
or depend only on the CPU (TEA, TERR, FWB) stay EL2's. A protected VM
still takes only TWI, TWE and VSE.

EL2 now sets TID2 or TID4 only for a protected VM, and never sets ATA,
as the host rejects KVM_CAP_ARM_MTE in pKVM.

Fixes: b56680de9c648 ("KVM: arm64: Initialize trap register values in hyp in pKVM")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/include/nvhe/pkvm.h |  9 +++++++++
 arch/arm64/kvm/hyp/nvhe/hyp-main.c     |  7 ++++---
 arch/arm64/kvm/hyp/nvhe/pkvm.c         | 18 ++++++++----------
 3 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
index c904647d2f760..5c050f21066ab 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/pkvm.h
@@ -12,6 +12,15 @@
 #include <nvhe/gfp.h>
 #include <nvhe/spinlock.h>
 
+/*
+ * HCR_EL2 bits EL2 takes from the host on each entry, per VM type. The rest
+ * are EL2's own and nothing the host sets there reaches the guest.
+ */
+#define PKVM_HCR_EL2_HOST_PVM	(HCR_EL2_TWI | HCR_EL2_TWE | HCR_EL2_VSE)
+#define PKVM_HCR_EL2_HOST_NPVM	(PKVM_HCR_EL2_HOST_PVM | HCR_EL2_VI | HCR_EL2_VF |	\
+				 HCR_EL2_TVM | HCR_EL2_TID2 | HCR_EL2_TID4 |		\
+				 HCR_EL2_TID5 | HCR_EL2_TTLBOS)
+
 /*
  * Holds the relevant data for maintaining the vcpu state completely at hyp.
  */
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 9a3b92e626adb..ac64a036b0a95 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -216,6 +216,7 @@ static void sync_debug_state(struct pkvm_hyp_vcpu *hyp_vcpu)
 static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
 {
 	struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu;
+	u64 host_hcr_mask = PKVM_HCR_EL2_HOST_PVM;
 
 	fpsimd_sve_flush();
 	flush_debug_state(hyp_vcpu);
@@ -228,6 +229,7 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
 	if (!pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
 		if (vcpu_get_flag(host_vcpu, PKVM_HOST_STATE_DIRTY))
 			flush_hyp_vcpu_state(hyp_vcpu);
+		host_hcr_mask = PKVM_HCR_EL2_HOST_NPVM;
 	} else {
 		hyp_vcpu->vcpu.arch.ctxt = host_vcpu->arch.ctxt;
 	}
@@ -241,9 +243,8 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
 	 * trap-control bit, so it must flow to the hyp vCPU alongside TWI/TWE
 	 * for the vSError to be delivered. sync_hyp_vcpu() reflects it back.
 	 */
-	hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE | HCR_VSE);
-	hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
-						 (HCR_TWI | HCR_TWE | HCR_VSE);
+	hyp_vcpu->vcpu.arch.hcr_el2 &= ~host_hcr_mask;
+	hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) & host_hcr_mask;
 
 	hyp_vcpu->vcpu.arch.iflags	= host_vcpu->arch.iflags;
 
diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 62d432144d44c..7ef09867f2802 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -30,6 +30,7 @@ unsigned int kvm_host_sve_max_vl;
  */
 static DEFINE_PER_CPU(struct pkvm_hyp_vcpu *, loaded_hyp_vcpu);
 
+/* The PKVM_HCR_EL2_HOST_{PVM,NPVM} bits of this value come from the host on each entry. */
 static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 {
 	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS;
@@ -47,13 +48,6 @@ static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
 		vcpu->arch.hcr_el2 |= HCR_FWB;
 
-	if (cpus_have_final_cap(ARM64_HAS_EVT) &&
-	    !cpus_have_final_cap(ARM64_MISMATCHED_CACHE_TYPE) &&
-	    kvm_read_vm_id_reg(vcpu->kvm, SYS_CTR_EL0) == read_cpuid(CTR_EL0))
-		vcpu->arch.hcr_el2 |= HCR_TID4;
-	else
-		vcpu->arch.hcr_el2 |= HCR_TID2;
-
 	/*
 	 * Without AArch32 EL1, leave RW set and let the entry fail with an
 	 * illegal exception return: the *32_EL2 registers EL2 would otherwise
@@ -65,9 +59,6 @@ static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 
 	if (vcpu_has_ptrauth(vcpu))
 		vcpu->arch.hcr_el2 |= (HCR_API | HCR_APK);
-
-	if (kvm_has_mte(vcpu->kvm))
-		vcpu->arch.hcr_el2 |= HCR_ATA;
 }
 
 static void pvm_init_traps_hcr(struct kvm_vcpu *vcpu)
@@ -85,6 +76,13 @@ static void pvm_init_traps_hcr(struct kvm_vcpu *vcpu)
 	 */
 	val |= HCR_TACR | HCR_TIDCP | HCR_TID3 | HCR_TID1;
 
+	if (cpus_have_final_cap(ARM64_HAS_EVT) &&
+	    !cpus_have_final_cap(ARM64_MISMATCHED_CACHE_TYPE) &&
+	    kvm_read_vm_id_reg(kvm, SYS_CTR_EL0) == read_cpuid(CTR_EL0))
+		val |= HCR_EL2_TID4;
+	else
+		val |= HCR_EL2_TID2;
+
 	if (!kvm_has_feat(kvm, ID_AA64PFR0_EL1, RAS, IMP)) {
 		val |= HCR_TERR | HCR_TEA;
 		val &= ~(HCR_FIEN);
-- 
2.39.5

[PATCH v3 4/4] KVM: arm64: selftests: Check a feature hidden in an ID register is UNDEF

[Fuad Tabba]

Userspace can hide a feature from a guest by clearing its field in a
writable ID register, and KVM then makes the feature's instructions
UNDEFINED in the guest by trapping or disabling them. No selftest checks
that.

Add a test that runs the instruction of each of TLBI OS, MOPS, TCR2_EL1
and FPMR once with its field as advertised and once with it cleared, and
expects an UNDEF only when cleared. A feature the vCPU doesn't advertise
is skipped, as is hidden TLBI OS on a CPU with neither FGT nor
FEAT_EVT2, where KVM can't trap it.

Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/arm64/hidden_features.c     | 184 ++++++++++++++++++
 2 files changed, 185 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/arm64/hidden_features.c

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 96bab7002d39e..864fdca7f362e 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -194,6 +194,7 @@ TEST_GEN_PROGS_arm64 += arm64/vgic_v5
 TEST_GEN_PROGS_arm64 += arm64/vpmu_counter_access
 TEST_GEN_PROGS_arm64 += arm64/no-vgic
 TEST_GEN_PROGS_arm64 += arm64/idreg-idst
+TEST_GEN_PROGS_arm64 += arm64/hidden_features
 TEST_GEN_PROGS_arm64 += arm64/kvm-uuid
 TEST_GEN_PROGS_arm64 += access_tracking_perf_test
 TEST_GEN_PROGS_arm64 += arch_timer
diff --git a/tools/testing/selftests/kvm/arm64/hidden_features.c b/tools/testing/selftests/kvm/arm64/hidden_features.c
new file mode 100644
index 0000000000000..194d746e7605e
--- /dev/null
+++ b/tools/testing/selftests/kvm/arm64/hidden_features.c
@@ -0,0 +1,184 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * hidden_features - Check that a feature's instruction runs in the guest when
+ * its ID register field is advertised, and is UNDEFINED when userspace clears
+ * the field.
+ *
+ * Copyright (c) 2026 Google LLC
+ * Author: Fuad Tabba <fuad.tabba@linux.dev>
+ */
+#include "kvm_util.h"
+#include "processor.h"
+#include "test_util.h"
+
+static volatile bool undef;
+
+static void guest_tlbi_os(void)
+{
+	/* tlbi vmalle1os */
+	asm volatile("sys #0, c8, c1, #0\n\tdsb ish\n\tisb" ::: "memory");
+}
+
+static void guest_mops(void)
+{
+	register u64 *d asm("x0");
+	register u64 n asm("x1");
+	register u64 s asm("x2");
+	u64 buf[8];
+
+	d = buf;
+	n = sizeof(buf);
+	s = 0;
+	/* setp [x0]!, x1!, x2; setm; sete */
+	asm volatile(".inst 0x19c20420\n\t.inst 0x19c24420\n\t.inst 0x19c28420"
+		     : "+r"(d), "+r"(n) : "r"(s) : "cc", "memory");
+}
+
+static void guest_tcr2(void)
+{
+	read_sysreg_s(SYS_TCR2_EL1);
+}
+
+static void guest_fpmr(void)
+{
+	read_sysreg_s(SYS_FPMR);
+}
+
+struct feature {
+	const char *name;
+	u64 id_reg;
+	u64 mask;
+	u8 shift;
+	u64 min;
+	void (*insn)(void);
+	bool (*trappable)(struct kvm_vcpu *vcpu);
+};
+
+/* Without FGT, KVM traps a hidden TLBI OS only through HCR_EL2.TTLBOS (FEAT_EVT2). */
+static bool tlbi_os_trappable(struct kvm_vcpu *vcpu)
+{
+	u64 mmfr0 = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_ID_AA64MMFR0_EL1));
+	u64 mmfr2 = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_ID_AA64MMFR2_EL1));
+
+	return SYS_FIELD_GET(ID_AA64MMFR0_EL1, FGT, mmfr0) >= ID_AA64MMFR0_EL1_FGT_IMP ||
+	       SYS_FIELD_GET(ID_AA64MMFR2_EL1, EVT, mmfr2) >= ID_AA64MMFR2_EL1_EVT_TTLBxS;
+}
+
+#define FEATURE(n, reg, field, min_val, fn, trap)		\
+{								\
+	.name		= n,					\
+	.id_reg		= SYS_##reg,				\
+	.mask		= reg##_##field##_MASK,			\
+	.shift		= reg##_##field##_SHIFT,		\
+	.min		= reg##_##field##_##min_val,		\
+	.insn		= fn,					\
+	.trappable	= trap,					\
+}
+
+static const struct feature features[] = {
+	FEATURE("TLBI OS", ID_AA64ISAR0_EL1, TLB, OS, guest_tlbi_os, tlbi_os_trappable),
+	FEATURE("MOPS", ID_AA64ISAR2_EL1, MOPS, IMP, guest_mops, NULL),
+	FEATURE("TCR2_EL1", ID_AA64MMFR3_EL1, TCRX, IMP, guest_tcr2, NULL),
+	FEATURE("FPMR", ID_AA64PFR2_EL1, FPMR, IMP, guest_fpmr, NULL),
+};
+
+static void guest_code(const struct feature *feat)
+{
+	undef = false;
+	feat->insn();
+	GUEST_SYNC(undef);
+	GUEST_DONE();
+}
+
+static void guest_undef_handler(struct ex_regs *regs)
+{
+	undef = true;
+	regs->pc += 4;
+}
+
+static bool run(const struct feature *feat, bool hide)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	struct ucall uc;
+	bool got = false;
+	u64 val;
+
+	vm = vm_create_with_one_vcpu(&vcpu, (void *)guest_code);
+	vm_init_descriptor_tables(vm);
+	vcpu_init_descriptor_tables(vcpu);
+	vm_install_sync_handler(vm, VECTOR_SYNC_CURRENT, ESR_ELx_EC_UNKNOWN, guest_undef_handler);
+	vcpu_args_set(vcpu, 1, feat);
+
+	if (hide) {
+		val = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(feat->id_reg));
+		vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(feat->id_reg), val & ~feat->mask);
+	}
+
+	for (;;) {
+		vcpu_run(vcpu);
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_SYNC:
+			got = uc.args[1];
+			break;
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+			break;
+		case UCALL_DONE:
+			kvm_vm_free(vm);
+			return got;
+		default:
+			TEST_FAIL("Unknown ucall %lu", uc.cmd);
+		}
+	}
+}
+
+static void probe_feature(const struct feature *feat, bool *present, bool *trappable)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	u64 val;
+
+	vm = vm_create_with_one_vcpu(&vcpu, NULL);
+	val = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(feat->id_reg));
+	*present = ((val & feat->mask) >> feat->shift) >= feat->min;
+	*trappable = !feat->trappable || feat->trappable(vcpu);
+	kvm_vm_free(vm);
+}
+
+int main(void)
+{
+	const struct feature *feat;
+	bool present, trappable;
+	int i;
+
+	test_disable_default_vgic();
+
+	ksft_print_header();
+	ksft_set_plan(ARRAY_SIZE(features) * 2);
+
+	for (i = 0; i < ARRAY_SIZE(features); i++) {
+		feat = &features[i];
+
+		probe_feature(feat, &present, &trappable);
+		if (!present) {
+			ksft_test_result_skip("%s advertised, not supported\n", feat->name);
+			ksft_test_result_skip("%s hidden, not supported\n", feat->name);
+			continue;
+		}
+
+		if (run(feat, false))
+			ksft_test_result_fail("%s advertised, UNDEF\n", feat->name);
+		else
+			ksft_test_result_pass("%s advertised\n", feat->name);
+
+		if (!trappable)
+			ksft_test_result_skip("%s hidden, not trappable\n", feat->name);
+		else if (run(feat, true))
+			ksft_test_result_pass("%s hidden\n", feat->name);
+		else
+			ksft_test_result_fail("%s hidden, no UNDEF\n", feat->name);
+	}
+
+	ksft_finished();
+}
-- 
2.39.5

Re: [PATCH v3 0/4] KVM: arm64: Fix HCR_EL2 for non-protected VMs in pKVM

[Oliver Upton]

On Tue, 29 Sep 2026 10:00:27 +0100, Fuad Tabba wrote:
> Changes since v2 [1]:
> - Patch 1: apply the FGUs on a CPU without FEAT_FGT instead of moving
>   the check in handle_tlbi_el1() (Oliver [2]). Wei-Lin's Reviewed-by
>   dropped with the rewrite.
> 
> In pKVM, EL2 sets a non-protected VM's HCR_EL2 in pkvm_vcpu_reset_hcr(),
> which misses the RW, TID5 and TTLBOS handling of vcpu_set_hcr(), and
> takes only TWI, TWE and VSE from the host. As a result, an AArch32 VM
> can't run, a VM can read GMID_EL1 or execute a TLBI OS its ID registers
> hide, and the host's TVM, VI and VF never reach it.
> 
> [...]

Applied to fixes, thanks!

[1/4] KVM: arm64: Apply the fine-grained UNDEFs without FEAT_FGT
      https://git.kernel.org/kvmarm/kvmarm/c/4bdd2b3a708c
[2/4] KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs
      https://git.kernel.org/kvmarm/kvmarm/c/80ae56184b57
[3/4] KVM: arm64: Use the host's HCR_EL2 for non-protected VMs in pKVM
      https://git.kernel.org/kvmarm/kvmarm/c/04447b95c62b
[4/4] KVM: arm64: selftests: Check a feature hidden in an ID register is UNDEF
      https://git.kernel.org/kvmarm/kvmarm/c/afb4334fb52c

--
Best,
Oliver