linux-kernel - [PATCH v3 21/23] KVM: s390: Enable 1M pages for gmap

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Message-ID: <20251106161117.350395-22-imbrenda@linux.ibm.com>
Date: Thu,  6 Nov 2025 17:11:15 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, nsg@...ux.ibm.com,
        nrb@...ux.ibm.com, seiden@...ux.ibm.com, schlameuss@...ux.ibm.com,
        hca@...ux.ibm.com, svens@...ux.ibm.com, agordeev@...ux.ibm.com,
        gor@...ux.ibm.com, david@...hat.com, gerald.schaefer@...ux.ibm.com
Subject: [PATCH v3 21/23] KVM: s390: Enable 1M pages for gmap

While userspace is allowed to have pages of any size, the new gmap
would always use 4k pages to back the guest.

Enable 1M pages for gmap.

This allows 1M pages to be used to back a guest when userspace is using
1M pages for the corresponding addresses (e.g. THP or hugetlbfs).

Remove the limitation that disallowed having nested guests and
hugepages at the same time.

Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
---
 arch/s390/kvm/gmap.c     | 2 +-
 arch/s390/kvm/kvm-s390.c | 6 +-----
 arch/s390/kvm/pv.c       | 3 +++
 3 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 067683da6cae..c4e759bc82b3 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -591,7 +591,7 @@ static inline bool gmap_2g_allowed(struct gmap *gmap, gfn_t gfn)
 
 static inline bool gmap_1m_allowed(struct gmap *gmap, gfn_t gfn)
 {
-	return false;
+	return gmap->allow_hpage_1m;
 }
 
 int gmap_link(struct kvm_s390_mmu_cache *mc, struct gmap *gmap, struct guest_fault *f)
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index fb92e9ba45bb..391644a88b4e 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -849,6 +849,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
 			r = -EINVAL;
 		else {
 			r = 0;
+			WRITE_ONCE(kvm->arch.gmap->allow_hpage_1m, 1);
 			/*
 			 * We might have to create fake 4k page
 			 * tables. To avoid that the hardware works on
@@ -5875,11 +5876,6 @@ static int __init kvm_s390_init(void)
 		return -ENODEV;
 	}
 
-	if (nested && hpage) {
-		pr_info("A KVM host that supports nesting cannot back its KVM guests with huge pages\n");
-		return -EINVAL;
-	}
-
 	for (i = 0; i < 16; i++)
 		kvm_s390_fac_base[i] |=
 			stfle_fac_list[i] & nonhyp_mask(i);
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
index d8a5c7b91148..8ea5f8d7e714 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -621,6 +621,9 @@ int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
 	uvcb.flags.ap_allow_instr = kvm->arch.model.uv_feat_guest.ap;
 	uvcb.flags.ap_instr_intr = kvm->arch.model.uv_feat_guest.ap_intr;
 
+	WRITE_ONCE(kvm->arch.gmap->allow_hpage_1m, 0);
+	gmap_split_huge_pages(kvm->arch.gmap);
+
 	cc = uv_call_sched(0, (u64)&uvcb);
 	*rc = uvcb.header.rc;
 	*rrc = uvcb.header.rrc;
-- 
2.51.1