KVM: MMU: do not write-protect large mappings

There is not much point in write protecting large mappings. This
can only happen when a page is shadowed during the window between
is_largepage_backed and mmu_lock acquision. Zap the entry instead, so
the next pagefault will find a shadowed page via is_largepage_backed and
fallback to 4k translations.

Simplifies out of sync shadow.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 23752ef0839c..731e6fe9cb07 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1180,11 +1180,16 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *shadow_pte,
             || (write_fault && !is_write_protection(vcpu) && !user_fault)) {
                 struct kvm_mmu_page *shadow;
 
+                if (largepage && has_wrprotected_page(vcpu->kvm, gfn)) {
+                        ret = 1;
+                        spte = shadow_trap_nonpresent_pte;
+                        goto set_pte;
+                }
+
                 spte |= PT_WRITABLE_MASK;
 
                 shadow = kvm_mmu_lookup_page(vcpu->kvm, gfn);
-                if (shadow ||
-                   (largepage && has_wrprotected_page(vcpu->kvm, gfn))) {
+                if (shadow) {
                         pgprintk("%s: found shadow page for %lx, marking ro\n",
                                  __func__, gfn);
                         ret = 1;
@@ -1197,6 +1202,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *shadow_pte,
         if (pte_access & ACC_WRITE_MASK)
                 mark_page_dirty(vcpu->kvm, gfn);
 
+set_pte:
         set_shadow_pte(shadow_pte, spte);
         return ret;
 }