[PATCH v6 27/28] KVM: s390: Enable 1M pages for gmap

Claudio Imbrenda
2025-12-22T16:51:19Z

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@linux.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 0abed178dde0..19392f3b398b 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -617,7 +617,7 @@ 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 test_bit(GMAP_FLAG_ALLOW_HPAGE_1M, &gmap->flags);
}

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 a714037cef31..47f2794af2fb 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -851,6 +851,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
r = -EINVAL;
else {
r = 0;
+ set_bit(GMAP_FLAG_ALLOW_HPAGE_1M, &kvm->arch.gmap->flags);
/*
* We might have to create fake 4k page
* tables. To avoid that the hardware works on
@@ -5739,11 +5740,6 @@ 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 b6809ee0bfa5..456c96a1c44a 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -721,6 +721,9 @@ 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;

+ clear_bit(GMAP_FLAG_ALLOW_HPAGE_1M, &kvm->arch.gmap->flags);
+ gmap_split_huge_pages(kvm->arch.gmap);
+
cc = uv_call_sched(0, (u64)&uvcb);
*rc = uvcb.header.rc;
*rrc = uvcb.header.rrc;
--
2.52.0