* Re: [PATCH] x86/hyper-v: Don't WARN on VM-Enter if eVMCS sees VMFUNC[63:32] != 0
2026-10-20 19:34 [PATCH] x86/hyper-v: Don't WARN on VM-Enter if eVMCS sees VMFUNC[63:32] != 0 Manuel Andreas
@ 2026-10-20 19:36 ` Manuel Andreas
2026-10-20 22:56 ` Sean Christopherson
1 sibling, 0 replies; 3+ messages in thread
From: Manuel Andreas @ 2026-10-20 19:36 UTC (permalink / raw)
To: kvm, linux-kernel, pbonzini, Sean Christopherson, Vitaly Kuznetsov
If relevant I utilized the following KVM unit test to trigger the WARN_ON_ONCE:
---
x86/unittests.cfg | 6 ++++++
x86/vmx.h | 1 +
x86/vmx_tests.c | 17 +++++++++++++++++
3 files changed, 24 insertions(+)
diff --git a/x86/unittests.cfg b/x86/unittests.cfg
index ae731a93..e46ecad1 100644
--- a/x86/unittests.cfg
+++ b/x86/unittests.cfg
@@ -482,6 +482,12 @@ arch = x86_64
groups = vmx nested_exception
check = /sys/module/kvm_intel/parameters/allow_smaller_maxphyaddr=Y
+[vmx_hyperv_vm_func_ctrls_warn]
+file = vmx.flat
+extra_params = -cpu max,+vmx,hv-vapic,hv-evmcs -append vmx_hyperv_vm_func_ctrls_warn
+arch = x86_64
+groups = vmx
+
[debug]
file = debug.flat
arch = x86_64
diff --git a/x86/vmx.h b/x86/vmx.h
index f27ecd72..8d26b1a5 100644
--- a/x86/vmx.h
+++ b/x86/vmx.h
@@ -207,6 +207,7 @@ enum Encoding {
APIC_VIRT_ADDR = 0x2012ul,
APIC_ACCS_ADDR = 0x2014ul,
POSTED_INTR_DESC_ADDR = 0x2016ul,
+ VM_FUNCTION_CONTROL = 0x2018ul,
EPTP = 0x201aul,
EPTP_HI = 0x201bul,
VMREAD_BITMAP = 0x2026ul,
diff --git a/x86/vmx_tests.c b/x86/vmx_tests.c
index 2c0feca0..287336be 100644
--- a/x86/vmx_tests.c
+++ b/x86/vmx_tests.c
@@ -12084,6 +12084,22 @@ static void vmx_cet_test(void)
test_set_guest_finished();
}
+static void vmx_hyperv_vm_func_ctrls_warn_guest(void) {
+ vmcall();
+}
+
+static void vmx_hyperv_vm_func_ctrls_warn(void)
+{
+ /* Set the unsupported bit */
+ vmcs_write(VM_FUNCTION_CONTROL, 1UL << 32UL);
+
+ test_set_guest(vmx_hyperv_vm_func_ctrls_warn_guest);
+ enter_guest();
+
+ test_set_guest_finished();
+ report(true, "hyperv warn on should have triggered!");
+}
+
#define TEST(name) { #name, .v2 = name }
/* name/init/guest_main/exit_handler/vmfail_handler */
@@ -12202,5 +12218,6 @@ struct vmx_test vmx_tests[] = {
TEST(vmx_canonical_test),
/* "Load CET" VM-entry/exit controls tests. */
TEST(vmx_cet_test),
+ TEST(vmx_hyperv_vm_func_ctrls_warn),
{ NULL, NULL, NULL, NULL },
};
--
2.47.3
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] x86/hyper-v: Don't WARN on VM-Enter if eVMCS sees VMFUNC[63:32] != 0
2026-10-20 19:34 [PATCH] x86/hyper-v: Don't WARN on VM-Enter if eVMCS sees VMFUNC[63:32] != 0 Manuel Andreas
2026-10-20 19:36 ` Manuel Andreas
@ 2026-10-20 22:56 ` Sean Christopherson
1 sibling, 0 replies; 3+ messages in thread
From: Sean Christopherson @ 2026-10-20 22:56 UTC (permalink / raw)
To: Manuel Andreas; +Cc: kvm, linux-kernel, pbonzini, Vitaly Kuznetsov
On Tue, Oct 20, 2026, Manuel Andreas wrote:
> With the necessary Hyper-V extensions for eVMCS enabled, an L1 guest can
> trivially fire a WARN_ON_ONCE in nested_evmcs_check_controls.
> While vmcs12->vm_function_control is technically already sanity checked in
> nested.c, that check is only performed if VMFUNC functionality is at all enabled
> in the secondary processor controls of vmcs12.
>
> vmcs12->vm_function_control seems to only be utilized in the handling
> of a VMFUNC initiated from L2, but here a function other than 0 will
> just end up in a failure case either way, regardless of whether or not
> it is permissive in vmcs12->vm_function_control.
> Therefore, the check has no reason to trigger a warning at all.
>
> This patch simply removes the WARN_ON_ONCE and keeps the check.
>
> Fixes: 3ff8a13d41b2 ("KVM: nVMX: WARN once and fail VM-Enter if eVMCS sees VMFUNC[63:32] != 0")
> Signed-off-by: Manuel Andreas <manuel.andreas@tum.de>
> ---
> arch/x86/kvm/vmx/hyperv.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
> index fa41d036acd4..008a06924eb4 100644
> --- a/arch/x86/kvm/vmx/hyperv.c
> +++ b/arch/x86/kvm/vmx/hyperv.c
> @@ -186,7 +186,7 @@ int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
> * controls in bits 63:32, i.e. dropping those bits on the consistency
> * check is intentional.
> */
> - if (WARN_ON_ONCE(vmcs12->vm_function_control >> 32))
I think the WARN is correct, this code is broken. Unless I'm missing some eVMCS
magic/quirk, this, and the secondary controls check, should be conditioned on the
field actually being used.
This, over two patches?
diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
index fa41d036acd4..a1daf8f2ec5a 100644
--- a/arch/x86/kvm/vmx/hyperv.c
+++ b/arch/x86/kvm/vmx/hyperv.c
@@ -169,7 +169,8 @@ int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
vmcs12->cpu_based_vm_exec_control)))
return -EINVAL;
- if (CC(!nested_evmcs_is_valid_controls(EVMCS_2NDEXEC,
+ if (nested_cpu_has(vmcs12, CPU_BASED_ACTIVATE_SECONDARY_CONTROLS) &&
+ CC(!nested_evmcs_is_valid_controls(EVMCS_2NDEXEC,
vmcs12->secondary_vm_exec_control)))
return -EINVAL;
@@ -186,11 +187,10 @@ int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
* controls in bits 63:32, i.e. dropping those bits on the consistency
* check is intentional.
*/
- if (WARN_ON_ONCE(vmcs12->vm_function_control >> 32))
- return -EINVAL;
-
- if (CC(!nested_evmcs_is_valid_controls(EVMCS_VMFUNC,
- vmcs12->vm_function_control)))
+ if (nested_cpu_has_vmfunc(vmcs12) &&
+ (WARN_ON_ONCE(vmcs12->vm_function_control >> 32) ||
+ CC(!nested_evmcs_is_valid_controls(EVMCS_VMFUNC,
+ vmcs12->vm_function_control))))
return -EINVAL;
return 0;
> + if (vmcs12->vm_function_control >> 32)
> return -EINVAL;
>
> if (CC(!nested_evmcs_is_valid_controls(EVMCS_VMFUNC,
^ permalink raw reply [flat|nested] 3+ messages in thread