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KVM: SEV: Drop page refcount early during RMP fault handling

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Rework KVM's handling of RMP faults to rely on MMU invalidation logic for
safety, instead of the current approach of holding onto a folio reference
until the RMP operations are complete.  I.e. drop the reference gifted by
guest_memfd immediately after getting the PFN, and instead do RMP updates
under mmu_lock, after checking for relevant MMU invalidations.

This will allow dropping guest_memfd's reference gifting entirely, which is
ideally how KVM would operate for all "follow PFN" operations (GUP has many
more complications, which is why KVM holds a reference across page
faults *on top* of the standard MMU invalidation logic).

Reviewed-by: Michael Roth <michael.roth@amd.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260826-gmem-no-return-page-v4-2-3bb9c1ddb4e3@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2026-08-26 09:01:47 +00:00
parent 2116065326
commit 375b32931b
1 changed files with 24 additions and 15 deletions
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39
arch/x86/kvm/svm/sev.c
View File

@ -5019,6 +5019,7 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
struct kvm_memory_slot *slot;
struct kvm *kvm = vcpu->kvm;
int order, rmp_level, ret;
unsigned long mmu_seq;
struct page *page;
bool assigned;
kvm_pfn_t pfn;
@ -5046,18 +5047,22 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
return;
}
mmu_seq = kvm->mmu_invalidate_seq;
smp_rmb();
ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &order);
if (ret) {
pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
gpa);
return;
}
kvm_release_page_unused(page);
ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
if (ret || !assigned) {
pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
gpa, pfn, ret);
goto out_no_trace;
return;
}
/*
@ -5085,27 +5090,31 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
if (rmp_level == PG_LEVEL_4K)
goto out;
ret = snp_rmptable_psmash(pfn);
if (ret) {
/*
* Look it up again. If it's 4K now then the PSMASH may have
* raced with another process and the issue has already resolved
* itself. If it's not assigned, then this must have raced with
* another process that made this page shared.
*/
if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
scoped_guard(read_lock, &kvm->mmu_lock) {
if (mmu_invalidate_retry_gfn(kvm, mmu_seq, gfn))
goto out;
pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
gpa, pfn, ret);
ret = snp_rmptable_psmash(pfn);
if (ret) {
/*
* Look it up again. If it's 4K now then the PSMASH may
* have raced with another process and the issue has
* already resolved itself. If it's not assigned, then
* this must have raced with another process that made
* this page shared.
*/
if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
goto out;
pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
gpa, pfn, ret);
}
}
kvm_zap_gfn_range(kvm, gfn, gfn + PTRS_PER_PMD);
out:
trace_kvm_rmp_fault(vcpu, gpa, pfn, error_code, rmp_level, ret);
out_no_trace:
kvm_release_page_unused(page);
}
static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)