[PATCH v2 19/20] KVM: s390: Enable 1M pages for gmap

Claudio Imbrenda
2025-09-10T18:07:59Z

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 001b069a6c7b..42914de5c791 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -599,7 +599,7 @@ 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 gmap *gmap, kvm_pfn_t pfn, struct page *page, gfn_t gfn, bool w, bool d)
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 1d6c15601a20..52b1709f0423 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -847,6 +847,7 @@ 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
@@ -5903,11 +5904,6 @@ 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 7da8aa17458a..1083d18c5dfc 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -618,6 +618,9 @@ 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;
--
2.51.0