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authorPaolo Bonzini <pbonzini@redhat.com>2022-05-03 07:23:08 -0400
committerPaolo Bonzini <pbonzini@redhat.com>2022-05-03 07:23:08 -0400
commit4f510c8bb1dd0edc5f8f82cbe990c6174ceb5a06 (patch)
tree978126728b3e43c9639a1b305be872d3a6567756 /arch
parentf751d8eac17692905cdd6935f72d523d8adf3b65 (diff)
parentba3a6120a4e7efc13d19fe43eb6c5caf1da05b72 (diff)
Merge branch 'kvm-tdp-mmu-atomicity-fix' into HEAD
We are dropping A/D bits (and W bits) in the TDP MMU. Even if mmu_lock is held for write, as volatile SPTEs can be written by other tasks/vCPUs outside of mmu_lock. Attempting to prove that bug exposed another notable goof, which has been lurking for a decade, give or take: KVM treats _all_ MMU-writable SPTEs as volatile, even though KVM never clears WRITABLE outside of MMU lock. As a result, the legacy MMU (and the TDP MMU if not fixed) uses XCHG to update writable SPTEs. The fix does not seem to have an easily-measurable affect on performance; page faults are so slow that wasting even a few hundred cycles is dwarfed by the base cost.
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kvm/mmu/mmu.c34
-rw-r--r--arch/x86/kvm/mmu/spte.c28
-rw-r--r--arch/x86/kvm/mmu/spte.h4
-rw-r--r--arch/x86/kvm/mmu/tdp_iter.h34
-rw-r--r--arch/x86/kvm/mmu/tdp_mmu.c82
5 files changed, 121 insertions, 61 deletions
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 64a2a7e2be90..311e4e1d7870 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -473,30 +473,6 @@ retry:
}
#endif
-static bool spte_has_volatile_bits(u64 spte)
-{
- if (!is_shadow_present_pte(spte))
- return false;
-
- /*
- * Always atomically update spte if it can be updated
- * out of mmu-lock, it can ensure dirty bit is not lost,
- * also, it can help us to get a stable is_writable_pte()
- * to ensure tlb flush is not missed.
- */
- if (spte_can_locklessly_be_made_writable(spte) ||
- is_access_track_spte(spte))
- return true;
-
- if (spte_ad_enabled(spte)) {
- if ((spte & shadow_accessed_mask) == 0 ||
- (is_writable_pte(spte) && (spte & shadow_dirty_mask) == 0))
- return true;
- }
-
- return false;
-}
-
/* Rules for using mmu_spte_set:
* Set the sptep from nonpresent to present.
* Note: the sptep being assigned *must* be either not present
@@ -557,7 +533,7 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
* we always atomically update it, see the comments in
* spte_has_volatile_bits().
*/
- if (spte_can_locklessly_be_made_writable(old_spte) &&
+ if (is_mmu_writable_spte(old_spte) &&
!is_writable_pte(new_spte))
flush = true;
@@ -591,7 +567,8 @@ static int mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
u64 old_spte = *sptep;
int level = sptep_to_sp(sptep)->role.level;
- if (!spte_has_volatile_bits(old_spte))
+ if (!is_shadow_present_pte(old_spte) ||
+ !spte_has_volatile_bits(old_spte))
__update_clear_spte_fast(sptep, 0ull);
else
old_spte = __update_clear_spte_slow(sptep, 0ull);
@@ -1187,7 +1164,7 @@ static bool spte_write_protect(u64 *sptep, bool pt_protect)
u64 spte = *sptep;
if (!is_writable_pte(spte) &&
- !(pt_protect && spte_can_locklessly_be_made_writable(spte)))
+ !(pt_protect && is_mmu_writable_spte(spte)))
return false;
rmap_printk("spte %p %llx\n", sptep, *sptep);
@@ -3196,8 +3173,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
* be removed in the fast path only if the SPTE was
* write-protected for dirty-logging or access tracking.
*/
- if (fault->write &&
- spte_can_locklessly_be_made_writable(spte)) {
+ if (fault->write && is_mmu_writable_spte(spte)) {
new_spte |= PT_WRITABLE_MASK;
/*
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4739b53c9734..e5c0b6db6f2c 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -90,6 +90,34 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
E820_TYPE_RAM);
}
+/*
+ * Returns true if the SPTE has bits that may be set without holding mmu_lock.
+ * The caller is responsible for checking if the SPTE is shadow-present, and
+ * for determining whether or not the caller cares about non-leaf SPTEs.
+ */
+bool spte_has_volatile_bits(u64 spte)
+{
+ /*
+ * Always atomically update spte if it can be updated
+ * out of mmu-lock, it can ensure dirty bit is not lost,
+ * also, it can help us to get a stable is_writable_pte()
+ * to ensure tlb flush is not missed.
+ */
+ if (!is_writable_pte(spte) && is_mmu_writable_spte(spte))
+ return true;
+
+ if (is_access_track_spte(spte))
+ return true;
+
+ if (spte_ad_enabled(spte)) {
+ if (!(spte & shadow_accessed_mask) ||
+ (is_writable_pte(spte) && !(spte & shadow_dirty_mask)))
+ return true;
+ }
+
+ return false;
+}
+
bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
const struct kvm_memory_slot *slot,
unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index e4abeb5df1b1..80ab0f5cff01 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -390,7 +390,7 @@ static inline void check_spte_writable_invariants(u64 spte)
"kvm: Writable SPTE is not MMU-writable: %llx", spte);
}
-static inline bool spte_can_locklessly_be_made_writable(u64 spte)
+static inline bool is_mmu_writable_spte(u64 spte)
{
return spte & shadow_mmu_writable_mask;
}
@@ -404,6 +404,8 @@ static inline u64 get_mmio_spte_generation(u64 spte)
return gen;
}
+bool spte_has_volatile_bits(u64 spte);
+
bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
const struct kvm_memory_slot *slot,
unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index b1eaf6ec0e0b..f0af385c56e0 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -6,6 +6,7 @@
#include <linux/kvm_host.h>
#include "mmu.h"
+#include "spte.h"
/*
* TDP MMU SPTEs are RCU protected to allow paging structures (non-leaf SPTEs)
@@ -17,9 +18,38 @@ static inline u64 kvm_tdp_mmu_read_spte(tdp_ptep_t sptep)
{
return READ_ONCE(*rcu_dereference(sptep));
}
-static inline void kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 val)
+
+static inline u64 kvm_tdp_mmu_write_spte_atomic(tdp_ptep_t sptep, u64 new_spte)
+{
+ return xchg(rcu_dereference(sptep), new_spte);
+}
+
+static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
+{
+ WRITE_ONCE(*rcu_dereference(sptep), new_spte);
+}
+
+static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
+ u64 new_spte, int level)
{
- WRITE_ONCE(*rcu_dereference(sptep), val);
+ /*
+ * Atomically write the SPTE if it is a shadow-present, leaf SPTE with
+ * volatile bits, i.e. has bits that can be set outside of mmu_lock.
+ * The Writable bit can be set by KVM's fast page fault handler, and
+ * Accessed and Dirty bits can be set by the CPU.
+ *
+ * Note, non-leaf SPTEs do have Accessed bits and those bits are
+ * technically volatile, but KVM doesn't consume the Accessed bit of
+ * non-leaf SPTEs, i.e. KVM doesn't care if it clobbers the bit. This
+ * logic needs to be reassessed if KVM were to use non-leaf Accessed
+ * bits, e.g. to skip stepping down into child SPTEs when aging SPTEs.
+ */
+ if (is_shadow_present_pte(old_spte) && is_last_spte(old_spte, level) &&
+ spte_has_volatile_bits(old_spte))
+ return kvm_tdp_mmu_write_spte_atomic(sptep, new_spte);
+
+ __kvm_tdp_mmu_write_spte(sptep, new_spte);
+ return old_spte;
}
/*
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index edc68538819b..922b06bf4b94 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -426,9 +426,9 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
tdp_mmu_unlink_sp(kvm, sp, shared);
for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
- u64 *sptep = rcu_dereference(pt) + i;
+ tdp_ptep_t sptep = pt + i;
gfn_t gfn = base_gfn + i * KVM_PAGES_PER_HPAGE(level);
- u64 old_child_spte;
+ u64 old_spte;
if (shared) {
/*
@@ -440,8 +440,8 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
* value to the removed SPTE value.
*/
for (;;) {
- old_child_spte = xchg(sptep, REMOVED_SPTE);
- if (!is_removed_spte(old_child_spte))
+ old_spte = kvm_tdp_mmu_write_spte_atomic(sptep, REMOVED_SPTE);
+ if (!is_removed_spte(old_spte))
break;
cpu_relax();
}
@@ -455,23 +455,43 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
* are guarded by the memslots generation, not by being
* unreachable.
*/
- old_child_spte = READ_ONCE(*sptep);
- if (!is_shadow_present_pte(old_child_spte))
+ old_spte = kvm_tdp_mmu_read_spte(sptep);
+ if (!is_shadow_present_pte(old_spte))
continue;
/*
- * Marking the SPTE as a removed SPTE is not
- * strictly necessary here as the MMU lock will
- * stop other threads from concurrently modifying
- * this SPTE. Using the removed SPTE value keeps
- * the two branches consistent and simplifies
- * the function.
+ * Use the common helper instead of a raw WRITE_ONCE as
+ * the SPTE needs to be updated atomically if it can be
+ * modified by a different vCPU outside of mmu_lock.
+ * Even though the parent SPTE is !PRESENT, the TLB
+ * hasn't yet been flushed, and both Intel and AMD
+ * document that A/D assists can use upper-level PxE
+ * entries that are cached in the TLB, i.e. the CPU can
+ * still access the page and mark it dirty.
+ *
+ * No retry is needed in the atomic update path as the
+ * sole concern is dropping a Dirty bit, i.e. no other
+ * task can zap/remove the SPTE as mmu_lock is held for
+ * write. Marking the SPTE as a removed SPTE is not
+ * strictly necessary for the same reason, but using
+ * the remove SPTE value keeps the shared/exclusive
+ * paths consistent and allows the handle_changed_spte()
+ * call below to hardcode the new value to REMOVED_SPTE.
+ *
+ * Note, even though dropping a Dirty bit is the only
+ * scenario where a non-atomic update could result in a
+ * functional bug, simply checking the Dirty bit isn't
+ * sufficient as a fast page fault could read the upper
+ * level SPTE before it is zapped, and then make this
+ * target SPTE writable, resume the guest, and set the
+ * Dirty bit between reading the SPTE above and writing
+ * it here.
*/
- WRITE_ONCE(*sptep, REMOVED_SPTE);
+ old_spte = kvm_tdp_mmu_write_spte(sptep, old_spte,
+ REMOVED_SPTE, level);
}
handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
- old_child_spte, REMOVED_SPTE, level,
- shared);
+ old_spte, REMOVED_SPTE, level, shared);
}
call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
@@ -667,14 +687,13 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
KVM_PAGES_PER_HPAGE(iter->level));
/*
- * No other thread can overwrite the removed SPTE as they
- * must either wait on the MMU lock or use
- * tdp_mmu_set_spte_atomic which will not overwrite the
- * special removed SPTE value. No bookkeeping is needed
- * here since the SPTE is going from non-present
- * to non-present.
+ * No other thread can overwrite the removed SPTE as they must either
+ * wait on the MMU lock or use tdp_mmu_set_spte_atomic() which will not
+ * overwrite the special removed SPTE value. No bookkeeping is needed
+ * here since the SPTE is going from non-present to non-present. Use
+ * the raw write helper to avoid an unnecessary check on volatile bits.
*/
- kvm_tdp_mmu_write_spte(iter->sptep, 0);
+ __kvm_tdp_mmu_write_spte(iter->sptep, 0);
return 0;
}
@@ -699,10 +718,13 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
* unless performing certain dirty logging operations.
* Leaving record_dirty_log unset in that case prevents page
* writes from being double counted.
+ *
+ * Returns the old SPTE value, which _may_ be different than @old_spte if the
+ * SPTE had voldatile bits.
*/
-static void __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
- u64 old_spte, u64 new_spte, gfn_t gfn, int level,
- bool record_acc_track, bool record_dirty_log)
+static u64 __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
+ u64 old_spte, u64 new_spte, gfn_t gfn, int level,
+ bool record_acc_track, bool record_dirty_log)
{
lockdep_assert_held_write(&kvm->mmu_lock);
@@ -715,7 +737,7 @@ static void __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
*/
WARN_ON(is_removed_spte(old_spte) || is_removed_spte(new_spte));
- kvm_tdp_mmu_write_spte(sptep, new_spte);
+ old_spte = kvm_tdp_mmu_write_spte(sptep, old_spte, new_spte, level);
__handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level, false);
@@ -724,6 +746,7 @@ static void __tdp_mmu_set_spte(struct kvm *kvm, int as_id, tdp_ptep_t sptep,
if (record_dirty_log)
handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
new_spte, level);
+ return old_spte;
}
static inline void _tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
@@ -732,9 +755,10 @@ static inline void _tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
{
WARN_ON_ONCE(iter->yielded);
- __tdp_mmu_set_spte(kvm, iter->as_id, iter->sptep, iter->old_spte,
- new_spte, iter->gfn, iter->level,
- record_acc_track, record_dirty_log);
+ iter->old_spte = __tdp_mmu_set_spte(kvm, iter->as_id, iter->sptep,
+ iter->old_spte, new_spte,
+ iter->gfn, iter->level,
+ record_acc_track, record_dirty_log);
}
static inline void tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,