[PATCH] KVM swapping with MMU Notifiers V7

Andrea Arcangeli
2008-02-16T10:48:27Z

Those below two patches enable KVM to swap the guest physical memory
through Christoph's V7.

There's one last _purely_theoretical_ race condition I figured out and
that I'm wondering how to best fix. The race condition worst case is
that a few guest physical pages could remain pinned by sptes. The race
can materialize if the linux pte is zapped after get_user_pages
returns but before the page is mapped by the spte and tracked by
rmap. The invalidate_ calls can also likely be optimized further but
it's not a fast path so it's not urgent.

Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 41962e7..e1287ab 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -21,6 +21,7 @@ config KVM
tristate "Kernel-based Virtual Machine (KVM) support"
depends on HAVE_KVM && EXPERIMENTAL
select PREEMPT_NOTIFIERS
+ select MMU_NOTIFIER
select ANON_INODES
---help---
Support hosting fully virtualized guest machines using hardware
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index fd39cd1..b56e388 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -533,6 +533,110 @@ static void rmap_write_protect(struct kvm *kvm, u64 gfn)
kvm_flush_remote_tlbs(kvm);
}

+static void kvm_unmap_spte(struct kvm *kvm, u64 *spte)
+{
+ struct page *page = pfn_to_page((*spte & PT64_BASE_ADDR_MASK) >> PAGE_SHIFT);
+ get_page(page);
+ rmap_remove(kvm, spte);
+ set_shadow_pte(spte, shadow_trap_nonpresent_pte);
+ kvm_flush_remote_tlbs(kvm);
+ __free_page(page);
+}
+
+static void kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp)
+{
+ u64 *spte, *curr_spte;
+
+ spte = rmap_next(kvm, rmapp, NULL);
+ while (spte) {
+ BUG_ON(!(*spte & PT_PRESENT_MASK));
+ rmap_printk("kvm_rmap_unmap_hva: spte %p %llx\n", spte, *spte);
+ curr_spte = spte;
+ spte = rmap_next(kvm, rmapp, spte);
+ kvm_unmap_spte(kvm, curr_spte);
+ }
+}
+
+void kvm_unmap_hva(struct kvm *kvm, unsigned long hva)
+{
+ int i;
+
+ /*
+ * If mmap_sem isn't taken, we can look the memslots with only
+ * the mmu_lock by skipping over the slots with userspace_addr == 0.
+ */
+ spin_lock(&kvm->mmu_lock);
+ for (i = 0; i < kvm->nmemslots; i++) {
+ struct kvm_memory_slot *memslot = &kvm->memslots[i];
+ unsigned long start = memslot->userspace_addr;
+ unsigned long end;
+
+ /* mmu_lock protects userspace_addr */
+ if (!start)
+ continue;
+
+ end = start + (memslot->npages << PAGE_SHIFT);
+ if (hva >= start && hva < end) {
+ gfn_t gfn_offset = (hva - start) >> PAGE_SHIFT;
+ kvm_unmap_rmapp(kvm, &memslot->rmap[gfn_offset]);
+ }
+ }
+ spin_unlock(&kvm->mmu_lock);
+}
+
+static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp)
+{
+ u64 *spte;
+ int young = 0;
+
+ spte = rmap_next(kvm, rmapp, NULL);
+ while (spte) {
+ int _young;
+ u64 _spte = *spte;
+ BUG_ON(!(_spte & PT_PRESENT_MASK));
+ _young = _spte & PT_ACCESSED_MASK;
+ if (_young) {
+ young = !!_young;
+ set_shadow_pte(spte, _spte & ~PT_ACCESSED_MASK);
+ }
+ spte = rmap_next(kvm, rmapp, spte);
+ }
+ return young;
+}
+
+int kvm_age_hva(struct kvm *kvm, unsigned long hva)
+{
+ int i;
+ int young = 0;
+
+ /*
+ * If mmap_sem isn't taken, we can look the memslots with only
+ * the mmu_lock by skipping over the slots with userspace_addr == 0.
+ */
+ spin_lock(&kvm->mmu_lock);
+ for (i = 0; i < kvm->nmemslots; i++) {
+ struct kvm_memory_slot *memslot = &kvm->memslots[i];
+ unsigned long start = memslot->userspace_addr;
+ unsigned long end;
+
+ /* mmu_lock protects userspace_addr */
+ if (!start)
+ continue;
+
+ end = start + (memslot->npages << PAGE_SHIFT);
+ if (hva >= start && hva < end) {
+ gfn_t gfn_offset = (hva - start) >> PAGE_SHIFT;
+ young |= kvm_age_rmapp(kvm, &memslot->rmap[gfn_offset]);
+ }
+ }
+ spin_unlock(&kvm->mmu_lock);
+
+ if (young)
+ kvm_flush_remote_tlbs(kvm);
+
+ return young;
+}
+
#ifdef MMU_DEBUG
static int is_empty_shadow_page(u64 *spt)
{
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0c910c7..2b2398f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3185,6 +3185,46 @@ void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu)
free_page((unsigned long)vcpu->arch.pio_data);
}

+static inline struct kvm *mmu_notifier_to_kvm(struct mmu_notifier *mn)
+{
+ struct kvm_arch *kvm_arch;
+ kvm_arch = container_of(mn, struct kvm_arch, mmu_notifier);
+ return container_of(kvm_arch, struct kvm, arch);
+}
+
+void kvm_mmu_notifier_invalidate_page(struct mmu_notifier *mn,
+ struct mm_struct *mm,
+ unsigned long address)
+{
+ struct kvm *kvm = mmu_notifier_to_kvm(mn);
+ BUG_ON(mm != kvm->mm);
+ kvm_unmap_hva(kvm, address);
+}
+
+int kvm_mmu_notifier_age_page(struct mmu_notifier *mn,
+ struct mm_struct *mm,
+ unsigned long address)
+{
+ struct kvm *kvm = mmu_notifier_to_kvm(mn);
+ BUG_ON(mm != kvm->mm);
+ return kvm_age_hva(kvm, address);
+}
+
+void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
+ struct mm_struct *mm,
+ unsigned long start, unsigned long end,
+ int lock)
+{
+ for (; start < end; start += PAGE_SIZE)
+ kvm_mmu_notifier_invalidate_page(mn, mm, start);
+}
+
+static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
+ .invalidate_page = kvm_mmu_notifier_invalidate_page,
+ .age_page = kvm_mmu_notifier_age_page,
+ .invalidate_range_end = kvm_mmu_notifier_invalidate_range_end,
+};
+
struct kvm *kvm_arch_create_vm(void)
{
struct kvm *kvm = kzalloc(sizeof(struct kvm), GFP_KERNEL);
@@ -3194,6 +3234,9 @@ struct kvm *kvm_arch_create_vm(void)

INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);

+ kvm->arch.mmu_notifier.ops = &kvm_mmu_notifier_ops;
+ mmu_notifier_register(&kvm->arch.mmu_notifier, current->mm);
+
return kvm;
}

diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index da61255..11976c8 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -13,6 +13,7 @@

#include <linux/types.h>
#include <linux/mm.h>
+#include <linux/mmu_notifier.h>

#include <linux/kvm.h>
#include <linux/kvm_para.h>
@@ -287,6 +288,8 @@ struct kvm_arch{
int round_robin_prev_vcpu;
unsigned int tss_addr;
struct page *apic_access_page;
+
+ struct mmu_notifier mmu_notifier;
};

struct kvm_vm_stat {
@@ -404,6 +407,8 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu);
int kvm_mmu_setup(struct kvm_vcpu *vcpu);
void kvm_mmu_set_nonpresent_ptes(u64 trap_pte, u64 notrap_pte);

+void kvm_unmap_hva(struct kvm *kvm, unsigned long hva);
+int kvm_age_hva(struct kvm *kvm, unsigned long hva);
int kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
void kvm_mmu_zap_all(struct kvm *kvm);

This allows to browse the memslots with only the mmu_lock hold and
it should be applied along the above patch:

Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0c910c7..80b719d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3245,16 +3245,23 @@ int kvm_arch_set_memory_region(struct kvm *kvm,
*/
if (!user_alloc) {
if (npages && !old.rmap) {
+ unsigned long userspace_addr;
+
down_write(&current->mm->mmap_sem);
- memslot->userspace_addr = do_mmap(NULL, 0,
- npages * PAGE_SIZE,
- PROT_READ | PROT_WRITE,
- MAP_SHARED | MAP_ANONYMOUS,
- 0);
+ userspace_addr = do_mmap(NULL, 0,
+ npages * PAGE_SIZE,
+ PROT_READ | PROT_WRITE,
+ MAP_SHARED | MAP_ANONYMOUS,
+ 0);
up_write(&current->mm->mmap_sem);

- if (IS_ERR((void *)memslot->userspace_addr))
- return PTR_ERR((void *)memslot->userspace_addr);
+ if (IS_ERR((void *)userspace_addr))
+ return PTR_ERR((void *)userspace_addr);
+
+ /* set userspace_addr atomically for kvm_hva_to_rmapp */
+ spin_lock(&kvm->mmu_lock);
+ memslot->userspace_addr = userspace_addr;
+ spin_unlock(&kvm->mmu_lock);
} else {
if (!old.user_alloc && old.rmap) {
int ret;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index cf6df51..743c5c5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -299,7 +299,15 @@ int __kvm_set_memory_region(struct kvm *kvm,
memset(new.rmap, 0, npages * sizeof(*new.rmap));

new.user_alloc = user_alloc;
- new.userspace_addr = mem->userspace_addr;
+ /*
+ * hva_to_rmmap() serialzies with the mmu_lock and to be
+ * safe it has to ignore memslots with !user_alloc &&
+ * !userspace_addr.
+ */
+ if (user_alloc)
+ new.userspace_addr = mem->userspace_addr;
+ else
+ new.userspace_addr = 0;
}

/* Allocate page dirty bitmap if needed */
@@ -312,14 +320,18 @@ int __kvm_set_memory_region(struct kvm *kvm,
memset(new.dirty_bitmap, 0, dirty_bytes);
}

+ spin_lock(&kvm->mmu_lock);
if (mem->slot >= kvm->nmemslots)
kvm->nmemslots = mem->slot + 1;

*memslot = new;
+ spin_unlock(&kvm->mmu_lock);

r = kvm_arch_set_memory_region(kvm, mem, old, user_alloc);
if (r) {
+ spin_lock(&kvm->mmu_lock);
*memslot = old;
+ spin_unlock(&kvm->mmu_lock);
goto out_free;
}

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