[PATCH 4/7] KVM: x86/mmu: Zap only obsolete roots if a root shadow page is zapped

Sean Christopherson
2021-12-09T06:06:15Z

Zap only obsolete roots when responding to zapping a single root shadow
page. Because KVM keeps root_count elevated when stuffing a previous
root into its PGD cache, shadowing a 64-bit guest means that zapping any
root causes all vCPUs to reload all roots, even if their current root is
not affected by the zap.

For many kernels, zapping a single root is a frequent operation, e.g. in
Linux it happens whenever an mm is dropped, e.g. process exits, etc...

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 2 +
arch/x86/kvm/mmu.h | 1 +
arch/x86/kvm/mmu/mmu.c | 66 +++++++++++++++++++++++++++++----
arch/x86/kvm/x86.c | 4 +-
4 files changed, 64 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d5fede05eb5f..62e5e842b692 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -102,6 +102,8 @@
#define KVM_REQ_MSR_FILTER_CHANGED KVM_ARCH_REQ(29)
#define KVM_REQ_UPDATE_CPU_DIRTY_LOGGING \
KVM_ARCH_REQ_FLAGS(30, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
+#define KVM_REQ_MMU_FREE_OBSOLETE_ROOTS \
+ KVM_ARCH_REQ_FLAGS(31, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)

#define CR0_RESERVED_BITS \
(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index e9fbb2c8bbe2..923e0e95e7d7 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -79,6 +79,7 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,

int kvm_mmu_load(struct kvm_vcpu *vcpu);
void kvm_mmu_unload(struct kvm_vcpu *vcpu);
+void kvm_mmu_free_obsolete_roots(struct kvm_vcpu *vcpu);
void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu);
void kvm_mmu_sync_prev_roots(struct kvm_vcpu *vcpu);

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 31605cd3c09f..b6115d8ea696 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2345,7 +2345,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
struct list_head *invalid_list,
int *nr_zapped)
{
- bool list_unstable;
+ bool list_unstable, zapped_root = false;

trace_kvm_mmu_prepare_zap_page(sp);
++kvm->stat.mmu_shadow_zapped;
@@ -2387,14 +2387,20 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
* in kvm_mmu_zap_all_fast(). Note, is_obsolete_sp() also
* treats invalid shadow pages as being obsolete.
*/
- if (!is_obsolete_sp(kvm, sp))
- kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
+ zapped_root = !is_obsolete_sp(kvm, sp);
}

if (sp->lpage_disallowed)
unaccount_huge_nx_page(kvm, sp);

sp->role.invalid = 1;
+
+ /*
+ * Make the request to free obsolete roots after marking the root
+ * invalid, otherwise other vCPUs may not see it as invalid.
+ */
+ if (zapped_root)
+ kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_FREE_OBSOLETE_ROOTS);
return list_unstable;
}

@@ -3985,7 +3991,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
* previous root, then __kvm_mmu_prepare_zap_page() signals all vCPUs
* to reload even if no vCPU is actively using the root.
*/
- if (!sp && kvm_test_request(KVM_REQ_MMU_RELOAD, vcpu))
+ if (!sp && kvm_test_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
return true;

return fault->slot &&
@@ -4187,8 +4193,8 @@ static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
/*
* It's possible that the cached previous root page is obsolete because
* of a change in the MMU generation number. However, changing the
- * generation number is accompanied by KVM_REQ_MMU_RELOAD, which will
- * free the root set here and allocate a new one.
+ * generation number is accompanied by KVM_REQ_MMU_FREE_OBSOLETE_ROOTS,
+ * which will free the root set here and allocate a new one.
*/
kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);

@@ -5113,6 +5119,52 @@ void kvm_mmu_unload(struct kvm_vcpu *vcpu)
WARN_ON(VALID_PAGE(vcpu->arch.guest_mmu.root_hpa));
}

+static bool is_obsolete_root(struct kvm_vcpu *vcpu, hpa_t root_hpa)
+{
+ struct kvm_mmu_page *sp;
+
+ if (!VALID_PAGE(root_hpa))
+ return false;
+
+ /*
+ * When freeing obsolete roots, treat roots as obsolete if they don't
+ * have an associated shadow page. This does mean KVM will get false
+ * positives and free roots that don't strictly need to be freed, but
+ * such false positives are relatively rare:
+ *
+ * (a) only PAE paging and nested NPT has roots without shadow pages
+ * (b) remote reloads due to a memslot update obsoletes _all_ roots
+ * (c) KVM doesn't track previous roots for PAE paging, and the guest
+ * is unlikely to zap an in-use PGD.
+ */
+ sp = to_shadow_page(root_hpa);
+ return !sp || is_obsolete_sp(vcpu->kvm, sp);
+}
+
+static void __kvm_mmu_free_obsolete_roots(struct kvm_vcpu *vcpu,
+ struct kvm_mmu *mmu)
+{
+ unsigned long roots_to_free = 0;
+ int i;
+
+ if (is_obsolete_root(vcpu, mmu->root_hpa))
+ roots_to_free |= KVM_MMU_ROOT_CURRENT;
+
+ for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++) {
+ if (is_obsolete_root(vcpu, mmu->root_hpa))
+ roots_to_free |= KVM_MMU_ROOT_PREVIOUS(i);
+ }
+
+ if (roots_to_free)
+ kvm_mmu_free_roots(vcpu, mmu, roots_to_free);
+}
+
+void kvm_mmu_free_obsolete_roots(struct kvm_vcpu *vcpu)
+{
+ __kvm_mmu_free_obsolete_roots(vcpu, &vcpu->arch.root_mmu);
+ __kvm_mmu_free_obsolete_roots(vcpu, &vcpu->arch.guest_mmu);
+}
+
static bool need_remote_flush(u64 old, u64 new)
{
if (!is_shadow_present_pte(old))
@@ -5686,7 +5738,7 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
* Note: we need to do this under the protection of mmu_lock,
* otherwise, vcpu would purge shadow page but miss tlb flush.
*/
- kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
+ kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_FREE_OBSOLETE_ROOTS);

kvm_zap_obsolete_pages(kvm);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ca1f0350a868..35a4f07b4f40 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9714,8 +9714,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
goto out;
}
}
- if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
- kvm_mmu_unload(vcpu);
+ if (kvm_check_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
+ kvm_mmu_free_obsolete_roots(vcpu);
if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
__kvm_migrate_timers(vcpu);
if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))
--
2.34.1.400.ga245620fadb-goog