[PATCH v1 0/3] KVM: s390: Add support for 2G hugepages

[PATCH v1 0/3] KVM: s390: Add support for 2G hugepages

[Claudio Imbrenda]

Add support for 2G hugepages.

In theory, since kernel 7.0, userspace could already use 2G hugepages
to back guest memory, but KVM would use 4k or 1M pages in gmap to
actually map the guest.

This series allows KVM on s390 to actually use 2G hugepages in gmap to
map guest memory that is backed by 2G hugepages in userspace.

Nested guests remain possible.

Claudio Imbrenda (3):
  KVM: s390: Add module parameter to fence 2G hugepages
  KVM: s390: Add capability to support 2G hugepages
  KVM: s390: Allow for 2G hugepages

 arch/s390/kvm/gmap.c     | 24 +++++++++++++++++++++++-
 arch/s390/kvm/kvm-s390.c | 38 +++++++++++++++++++++++++++++++++++++-
 arch/s390/kvm/pv.c       |  1 +
 include/uapi/linux/kvm.h |  1 +
 4 files changed, 62 insertions(+), 2 deletions(-)

-- 
2.54.0

[PATCH v1 1/3] KVM: s390: Add module parameter to fence 2G hugepages

[Claudio Imbrenda]

Add the hpage_2g module parameter to KVM to allow enabling or disabling
2G hugepages in KVM.

If hpage_2g is enabled but hpage is not enabled, print a message and
disable hpage_2g.

Opportunistically fix the comment for the hpage module parameter.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/kvm-s390.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index ffb20a64d328..801a622691b6 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -204,11 +204,16 @@ static int nested;
 module_param(nested, int, S_IRUGO);
 MODULE_PARM_DESC(nested, "Nested virtualization support");
 
-/* allow 1m huge page guest backing, if !nested */
+/* allow 1m huge page guest backing */
 static int hpage;
 module_param(hpage, int, 0444);
 MODULE_PARM_DESC(hpage, "1m huge page backing support");
 
+/* allow 2g huge page guest backing */
+static int hpage_2g;
+module_param(hpage_2g, int, 0444);
+MODULE_PARM_DESC(hpage_2g, "2g huge page backing support");
+
 /* maximum percentage of steal time for polling.  >100 is treated like 100 */
 static u8 halt_poll_max_steal = 10;
 module_param(halt_poll_max_steal, byte, 0644);
@@ -5820,6 +5825,11 @@ static int __init kvm_s390_init(void)
 		return -ENODEV;
 	}
 
+	if (hpage_2g && !hpage) {
+		hpage_2g = 0;
+		pr_info("Disabling 2G hugepage support, since 1M hugepage support is not enabled.\n");
+	}
+
 	for (i = 0; i < 16; i++)
 		kvm_s390_fac_base[i] |=
 			stfle_fac_list[i] & nonhyp_mask(i);
-- 
2.54.0

[PATCH v1 2/3] KVM: s390: Add capability to support 2G hugepages

[Claudio Imbrenda]

Add KVM_CAP_S390_HPAGE_2G to signal to userspace that 2G hugepages may
be used to back the guest; restrictions apply similar to 1M hugepages.

Enable the (for now still ignored) GMAP_FLAG_ALLOW_HPAGE_2G flag for
the guest gmap, and propagate / disable it as necessary.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/gmap.c     |  5 +++++
 arch/s390/kvm/kvm-s390.c | 26 ++++++++++++++++++++++++++
 arch/s390/kvm/pv.c       |  1 +
 include/uapi/linux/kvm.h |  1 +
 4 files changed, 33 insertions(+)

diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 52d55ddea8d4..c9e348149ba1 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -105,6 +105,11 @@ static void gmap_add_child(struct gmap *parent, struct gmap *child)
 	else
 		clear_bit(GMAP_FLAG_ALLOW_HPAGE_1M, &child->flags);
 
+	if (test_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &parent->flags))
+		set_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &child->flags);
+	else
+		clear_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &child->flags);
+
 	if (kvm_is_ucontrol(parent->kvm))
 		clear_bit(GMAP_FLAG_OWNS_PAGETABLES, &child->flags);
 	list_add(&child->list, &parent->children);
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 801a622691b6..616d1db7c0d9 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -645,6 +645,11 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		if (hpage && !(kvm && kvm_is_ucontrol(kvm)))
 			r = 1;
 		break;
+	case KVM_CAP_S390_HPAGE_2G:
+		r = 0;
+		if (hpage_2g && !(kvm && kvm_is_ucontrol(kvm)))
+			r = 1;
+		break;
 	case KVM_CAP_S390_MEM_OP:
 		r = MEM_OP_MAX_SIZE;
 		break;
@@ -901,6 +906,27 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
 		VM_EVENT(kvm, 3, "ENABLE: CAP_S390_HPAGE %s",
 			 r ? "(not available)" : "(success)");
 		break;
+	case KVM_CAP_S390_HPAGE_2G:
+		mutex_lock(&kvm->lock);
+		if (kvm->created_vcpus) {
+			r = -EBUSY;
+		} else if (!hpage_2g || kvm->arch.use_cmma || kvm_is_ucontrol(kvm)) {
+			r = -EINVAL;
+		} else {
+			r = 0;
+			set_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &kvm->arch.gmap->flags);
+			/*
+			 * We might have to create fake 4k page
+			 * tables. To avoid that the hardware works on
+			 * stale PGSTEs, we emulate these instructions.
+			 */
+			kvm->arch.use_skf = 0;
+			kvm->arch.use_pfmfi = 0;
+		}
+		mutex_unlock(&kvm->lock);
+		VM_EVENT(kvm, 3, "ENABLE: CAP_S390_HPAGE_2G %s",
+			 r ? "(not available)" : "(success)");
+		break;
 	case KVM_CAP_S390_USER_STSI:
 		VM_EVENT(kvm, 3, "%s", "ENABLE: CAP_S390_USER_STSI");
 		kvm->arch.user_stsi = 1;
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
index 4b865e75351c..0f818149e644 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -741,6 +741,7 @@ int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
 	uvcb.flags.ap_instr_intr = kvm->arch.model.uv_feat_guest.ap_intr;
 
 	clear_bit(GMAP_FLAG_ALLOW_HPAGE_1M, &kvm->arch.gmap->flags);
+	clear_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &kvm->arch.gmap->flags);
 	gmap_split_huge_pages(kvm->arch.gmap);
 
 	cc = uv_call_sched(0, (u64)&uvcb);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 6c8afa2047bf..419011097fa8 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -996,6 +996,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_S390_USER_OPEREXEC 246
 #define KVM_CAP_S390_KEYOP 247
 #define KVM_CAP_S390_VSIE_ESAMODE 248
+#define KVM_CAP_S390_HPAGE_2G 249
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
-- 
2.54.0

[PATCH v1 3/3] KVM: s390: Allow for 2G hugepages

[Claudio Imbrenda]

Change gmap_2g_allowed() to perform the necessary checks to allow for
2G hugepages to be used, instead of returning false. The
GMAP_FLAG_ALLOW_HPAGE_2G gmap flag is now taken into account.

Also add appropriate kerneldoc comments.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/gmap.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index c9e348149ba1..c5a8132eaa22 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -635,10 +635,27 @@ int gmap_try_fixup_minor(struct gmap *gmap, struct guest_fault *fault)
 	return rc;
 }
 
+/**
+ * gmap_2g_allowed() - Check whether a 2G hugepage is allowed.
+ * @gmap: The gmap of the guest.
+ * @f: Describes the fault that is being resolved.
+ * @slot: The memslot the faulting address belongs to.
+ *
+ * The function checks whether the GMAP_FLAG_ALLOW_HPAGE_2G flag is set for
+ * @gmap, whether the offset of the address in the 2G virtual frame is the
+ * same as the offset in the physical 2G frame, and finally whether the whole
+ * 2G page would fit in the given memslot.
+ *
+ * Return: true if a 2G hugepage is allowed to back the faulting address, false
+ *         otherwise.
+ */
 static inline bool gmap_2g_allowed(struct gmap *gmap, struct guest_fault *f,
 				   struct kvm_memory_slot *slot)
 {
-	return false;
+	return test_bit(GMAP_FLAG_ALLOW_HPAGE_2G, &gmap->flags) &&
+	       !((f->gfn ^ f->pfn) & ~_REGION3_FR_MASK) &&
+	       slot->base_gfn <= ALIGN_DOWN(f->gfn, _PAGES_PER_REGION3) &&
+	       slot->base_gfn + slot->npages >= ALIGN(f->gfn + 1, _PAGES_PER_REGION3);
 }
 
 /**
-- 
2.54.0

Re: [PATCH v1 1/3] KVM: s390: Add module parameter to fence 2G hugepages

[Steffen Eiden]

On Wed, Jun 03, 2026 at 05:47:56PM +0200, Claudio Imbrenda wrote:
> Add the hpage_2g module parameter to KVM to allow enabling or disabling
> 2G hugepages in KVM.
> 
> If hpage_2g is enabled but hpage is not enabled, print a message and
> disable hpage_2g.
> 
> Opportunistically fix the comment for the hpage module parameter.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>

Re: [PATCH v1 2/3] KVM: s390: Add capability to support 2G hugepages

[Steffen Eiden]

On Wed, Jun 03, 2026 at 05:47:57PM +0200, Claudio Imbrenda wrote:
> Add KVM_CAP_S390_HPAGE_2G to signal to userspace that 2G hugepages may
> be used to back the guest; restrictions apply similar to 1M hugepages.
> 
> Enable the (for now still ignored) GMAP_FLAG_ALLOW_HPAGE_2G flag for
> the guest gmap, and propagate / disable it as necessary.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>

Re: [PATCH v1 3/3] KVM: s390: Allow for 2G hugepages

[Steffen Eiden]

On Wed, Jun 03, 2026 at 05:47:58PM +0200, Claudio Imbrenda wrote:
> Change gmap_2g_allowed() to perform the necessary checks to allow for
> 2G hugepages to be used, instead of returning false. The
> GMAP_FLAG_ALLOW_HPAGE_2G gmap flag is now taken into account.
> 
> Also add appropriate kerneldoc comments.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>