[PATCH 25/33] KVM: MMU: Never free a shadow page actively serving as a root

Avi Kivity
2007-01-04T16:14:07Z

We always need cr3 to point to something valid, so if we detect that we're
freeing a root page, simply push it back to the top of the active list.

Signed-off-by: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>

Index: linux-2.6/drivers/kvm/mmu.c
===================================================================
--- linux-2.6.orig/drivers/kvm/mmu.c
+++ linux-2.6/drivers/kvm/mmu.c
@@ -550,8 +550,13 @@ static void kvm_mmu_zap_page(struct kvm_
*parent_pte = 0;
}
kvm_mmu_page_unlink_children(vcpu, page);
- hlist_del(&page->hash_link);
- kvm_mmu_free_page(vcpu, page->page_hpa);
+ if (!page->root_count) {
+ hlist_del(&page->hash_link);
+ kvm_mmu_free_page(vcpu, page->page_hpa);
+ } else {
+ list_del(&page->link);
+ list_add(&page->link, &vcpu->kvm->active_mmu_pages);
+ }
}

static int kvm_mmu_unprotect_page(struct kvm_vcpu *vcpu, gfn_t gfn)
@@ -667,12 +672,15 @@ static int nonpaging_map(struct kvm_vcpu
static void mmu_free_roots(struct kvm_vcpu *vcpu)
{
int i;
+ struct kvm_mmu_page *page;

#ifdef CONFIG_X86_64
if (vcpu->mmu.shadow_root_level == PT64_ROOT_LEVEL) {
hpa_t root = vcpu->mmu.root_hpa;

ASSERT(VALID_PAGE(root));
+ page = page_header(root);
+ --page->root_count;
vcpu->mmu.root_hpa = INVALID_PAGE;
return;
}
@@ -682,6 +690,8 @@ static void mmu_free_roots(struct kvm_vc

ASSERT(VALID_PAGE(root));
root &= PT64_BASE_ADDR_MASK;
+ page = page_header(root);
+ --page->root_count;
vcpu->mmu.pae_root[i] = INVALID_PAGE;
}
vcpu->mmu.root_hpa = INVALID_PAGE;
@@ -691,6 +701,8 @@ static void mmu_alloc_roots(struct kvm_v
{
int i;
gfn_t root_gfn;
+ struct kvm_mmu_page *page;
+
root_gfn = vcpu->cr3 >> PAGE_SHIFT;

#ifdef CONFIG_X86_64
@@ -700,6 +712,8 @@ static void mmu_alloc_roots(struct kvm_v
ASSERT(!VALID_PAGE(root));
root = kvm_mmu_get_page(vcpu, root_gfn, 0,
PT64_ROOT_LEVEL, 0, NULL)->page_hpa;
+ page = page_header(root);
+ ++page->root_count;
vcpu->mmu.root_hpa = root;
return;
}
@@ -715,6 +729,8 @@ static void mmu_alloc_roots(struct kvm_v
root = kvm_mmu_get_page(vcpu, root_gfn, i << 30,
PT32_ROOT_LEVEL, !is_paging(vcpu),
NULL)->page_hpa;
+ page = page_header(root);
+ ++page->root_count;
vcpu->mmu.pae_root[i] = root | PT_PRESENT_MASK;
}
vcpu->mmu.root_hpa = __pa(vcpu->mmu.pae_root);
Index: linux-2.6/drivers/kvm/kvm.h
===================================================================
--- linux-2.6.orig/drivers/kvm/kvm.h
+++ linux-2.6/drivers/kvm/kvm.h
@@ -134,6 +134,7 @@ struct kvm_mmu_page {
*/
int global; /* Set if all ptes in this page are global */
int multimapped; /* More than one parent_pte? */
+ int root_count; /* Currently serving as active root */
union {
u64 *parent_pte; /* !multimapped */
struct hlist_head parent_ptes; /* multimapped, kvm_pte_chain */