[patch 05/13] KVM: MMU: do not write-protect large mappings

Marcelo Tosatti
2008-09-06T19:27:13Z

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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.

Index: kvm/arch/x86/kvm/mmu.c
===================================================================
--- kvm.orig/arch/x86/kvm/mmu.c
+++ kvm/arch/x86/kvm/mmu.c
@@ -1210,8 +1210,7 @@ static void mmu_set_spte(struct kvm_vcpu
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);
pte_access &= ~ACC_WRITE_MASK;
@@ -1222,6 +1221,14 @@ static void mmu_set_spte(struct kvm_vcpu
if (write_fault)
*ptwrite = 1;
}
+ /*
+ * Do not create write protected large translations.
+ */
+ if (largepage && has_wrprotected_page(vcpu->kvm, gfn)) {
+ spte = shadow_trap_nonpresent_pte;
+ was_writeble = 0;
+ *ptwrite = 0;
+ }
}

if (pte_access & ACC_WRITE_MASK)
Index: kvm/arch/x86/kvm/paging_tmpl.h
===================================================================
--- kvm.orig/arch/x86/kvm/paging_tmpl.h
+++ kvm/arch/x86/kvm/paging_tmpl.h
@@ -307,11 +307,10 @@ static int FNAME(shadow_walk_entry)(stru
return 1;
}

- if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep))
+ if (is_shadow_present_pte(*sptep))
return 0;

- if (is_large_pte(*sptep))
- rmap_remove(vcpu->kvm, sptep);
+ WARN_ON (is_large_pte(*sptep));

if (level == PT_DIRECTORY_LEVEL && gw->level == PT_DIRECTORY_LEVEL) {
metaphysical = 1;

--