Re: [PATCH v10 7/8] KVM: x86/mmu: Track if sptes refer to refcounted pages

From: Paolo Bonzini
Date: Fri Feb 23 2024 - 05:01:03 EST

On 2/21/24 08:25, David Stevens wrote:
From: David Stevens <stevensd@xxxxxxxxxxxx>

Use one of the unused bits in EPT sptes to track whether or not an spte
refers to a struct page that has a valid refcount, in preparation for
adding support for mapping such pages into guests. The new bit is used
to avoid triggering a page_count() == 0 warning and to avoid touching
A/D bits of unknown usage.

Non-EPT sptes don't have any free bits to use, so this tracking is not
possible when TDP is disabled or on 32-bit x86.

TDX will add support for non-zero non-present PTEs. We could use this to use inverted bit 8 to mark present PTEs (bit 8 set for non-present, bit 8 clear for present) for both shadow paging and AMD NPT. This would free bit 11 for SPTE_MMU_PAGE_REFCOUNTED.

No need to do it now, though.

Paolo

Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 43 +++++++++++++++++++---------------
arch/x86/kvm/mmu/paging_tmpl.h | 5 ++--
arch/x86/kvm/mmu/spte.c | 4 +++-
arch/x86/kvm/mmu/spte.h | 22 ++++++++++++++++-
arch/x86/kvm/mmu/tdp_mmu.c | 21 ++++++++++-------
include/linux/kvm_host.h | 3 +++
virt/kvm/kvm_main.c | 6 +++--
7 files changed, 70 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index bbeb0f6783d7..7c059b23ae16 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -541,12 +541,14 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
if (is_accessed_spte(old_spte) && !is_accessed_spte(new_spte)) {
flush = true;
- kvm_set_pfn_accessed(spte_to_pfn(old_spte));
+ if (is_refcounted_page_spte(old_spte))
+ kvm_set_page_accessed(pfn_to_page(spte_to_pfn(old_spte)));
}
if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte)) {
flush = true;
- kvm_set_pfn_dirty(spte_to_pfn(old_spte));
+ if (is_refcounted_page_spte(old_spte))
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(old_spte)));
}
return flush;
@@ -578,20 +580,23 @@ static u64 mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
pfn = spte_to_pfn(old_spte);
- /*
- * KVM doesn't hold a reference to any pages mapped into the guest, and
- * instead uses the mmu_notifier to ensure that KVM unmaps any pages
- * before they are reclaimed. Sanity check that, if the pfn is backed
- * by a refcounted page, the refcount is elevated.
- */
- page = kvm_pfn_to_refcounted_page(pfn);
- WARN_ON_ONCE(page && !page_count(page));
+ if (is_refcounted_page_spte(old_spte)) {
+ /*
+ * KVM doesn't hold a reference to any pages mapped into the
+ * guest, and instead uses the mmu_notifier to ensure that KVM
+ * unmaps any pages before they are reclaimed. Sanity check
+ * that, if the pfn is backed by a refcounted page, the
+ * refcount is elevated.
+ */
+ page = kvm_pfn_to_refcounted_page(pfn);
+ WARN_ON_ONCE(!page || !page_count(page));
- if (is_accessed_spte(old_spte))
- kvm_set_pfn_accessed(pfn);
+ if (is_accessed_spte(old_spte))
+ kvm_set_page_accessed(pfn_to_page(pfn));
- if (is_dirty_spte(old_spte))
- kvm_set_pfn_dirty(pfn);
+ if (is_dirty_spte(old_spte))
+ kvm_set_page_dirty(pfn_to_page(pfn));
+ }
return old_spte;
}
@@ -627,8 +632,8 @@ static bool mmu_spte_age(u64 *sptep)
* Capture the dirty status of the page, so that it doesn't get
* lost when the SPTE is marked for access tracking.
*/
- if (is_writable_pte(spte))
- kvm_set_pfn_dirty(spte_to_pfn(spte));
+ if (is_writable_pte(spte) && is_refcounted_page_spte(spte))
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(spte)));
spte = mark_spte_for_access_track(spte);
mmu_spte_update_no_track(sptep, spte);
@@ -1267,8 +1272,8 @@ static bool spte_wrprot_for_clear_dirty(u64 *sptep)
{
bool was_writable = test_and_clear_bit(PT_WRITABLE_SHIFT,
(unsigned long *)sptep);
- if (was_writable && !spte_ad_enabled(*sptep))
- kvm_set_pfn_dirty(spte_to_pfn(*sptep));
+ if (was_writable && !spte_ad_enabled(*sptep) && is_refcounted_page_spte(*sptep))
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(*sptep)));
return was_writable;
}
@@ -2946,7 +2951,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
}
wrprot = make_spte(vcpu, sp, slot, pte_access, gfn, pfn, *sptep, prefetch,
- true, host_writable, &spte);
+ true, host_writable, true, &spte);
if (*sptep == spte) {
ret = RET_PF_SPURIOUS;
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 4d4e98fe4f35..c965f77ac4d5 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -902,7 +902,7 @@ static gpa_t FNAME(gva_to_gpa)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
*/
static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int i)
{
- bool host_writable;
+ bool host_writable, is_refcounted;
gpa_t first_pte_gpa;
u64 *sptep, spte;
struct kvm_memory_slot *slot;
@@ -959,10 +959,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
sptep = &sp->spt[i];
spte = *sptep;
host_writable = spte & shadow_host_writable_mask;
+ is_refcounted = is_refcounted_page_spte(spte);
slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
make_spte(vcpu, sp, slot, pte_access, gfn,
spte_to_pfn(spte), spte, true, false,
- host_writable, &spte);
+ host_writable, is_refcounted, &spte);
return mmu_spte_update(sptep, spte);
}
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4a599130e9c9..efba85df6518 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -138,7 +138,7 @@ 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,
u64 old_spte, bool prefetch, bool can_unsync,
- bool host_writable, u64 *new_spte)
+ bool host_writable, bool is_refcounted, u64 *new_spte)
{
int level = sp->role.level;
u64 spte = SPTE_MMU_PRESENT_MASK;
@@ -188,6 +188,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
if (level > PG_LEVEL_4K)
spte |= PT_PAGE_SIZE_MASK;
+ if (spte_has_refcount_bit() && is_refcounted)
+ spte |= SPTE_MMU_PAGE_REFCOUNTED;
if (shadow_memtype_mask)
spte |= static_call(kvm_x86_get_mt_mask)(vcpu, gfn,
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index a129951c9a88..4101cc9ef52f 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -96,6 +96,11 @@ static_assert(!(EPT_SPTE_MMU_WRITABLE & SHADOW_ACC_TRACK_SAVED_MASK));
/* Defined only to keep the above static asserts readable. */
#undef SHADOW_ACC_TRACK_SAVED_MASK
+/*
+ * Indicates that the SPTE refers to a page with a valid refcount.
+ */
+#define SPTE_MMU_PAGE_REFCOUNTED BIT_ULL(59)
+
/*
* Due to limited space in PTEs, the MMIO generation is a 19 bit subset of
* the memslots generation and is derived as follows:
@@ -345,6 +350,21 @@ static inline bool is_dirty_spte(u64 spte)
return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
}
+/*
+ * Extra bits are only available for TDP SPTEs, since bits 62:52 are reserved
+ * for PAE paging, including NPT PAE. When a tracking bit isn't available, we
+ * will reject mapping non-refcounted struct pages.
+ */
+static inline bool spte_has_refcount_bit(void)
+{
+ return tdp_enabled && IS_ENABLED(CONFIG_X86_64);
+}
+
+static inline bool is_refcounted_page_spte(u64 spte)
+{
+ return !spte_has_refcount_bit() || (spte & SPTE_MMU_PAGE_REFCOUNTED);
+}
+
static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,
int level)
{
@@ -475,7 +495,7 @@ 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,
u64 old_spte, bool prefetch, bool can_unsync,
- bool host_writable, u64 *new_spte);
+ bool host_writable, bool is_refcounted, u64 *new_spte);
u64 make_huge_page_split_spte(struct kvm *kvm, u64 huge_spte,
union kvm_mmu_page_role role, int index);
u64 make_nonleaf_spte(u64 *child_pt, bool ad_disabled);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6ae19b4ee5b1..ee497fb78d90 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -414,6 +414,7 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
bool was_leaf = was_present && is_last_spte(old_spte, level);
bool is_leaf = is_present && is_last_spte(new_spte, level);
bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
+ bool is_refcounted = is_refcounted_page_spte(old_spte);
WARN_ON_ONCE(level > PT64_ROOT_MAX_LEVEL);
WARN_ON_ONCE(level < PG_LEVEL_4K);
@@ -478,9 +479,9 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
if (is_leaf != was_leaf)
kvm_update_page_stats(kvm, level, is_leaf ? 1 : -1);
- if (was_leaf && is_dirty_spte(old_spte) &&
+ if (was_leaf && is_dirty_spte(old_spte) && is_refcounted &&
(!is_present || !is_dirty_spte(new_spte) || pfn_changed))
- kvm_set_pfn_dirty(spte_to_pfn(old_spte));
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(old_spte)));
/*
* Recursively handle child PTs if the change removed a subtree from
@@ -492,9 +493,9 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
(is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
handle_removed_pt(kvm, spte_to_child_pt(old_spte, level), shared);
- if (was_leaf && is_accessed_spte(old_spte) &&
+ if (was_leaf && is_accessed_spte(old_spte) && is_refcounted &&
(!is_present || !is_accessed_spte(new_spte) || pfn_changed))
- kvm_set_pfn_accessed(spte_to_pfn(old_spte));
+ kvm_set_page_accessed(pfn_to_page(spte_to_pfn(old_spte)));
}
/*
@@ -956,8 +957,8 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
else
wrprot = make_spte(vcpu, sp, fault->slot, ACC_ALL, iter->gfn,
- fault->pfn, iter->old_spte, fault->prefetch, true,
- fault->map_writable, &new_spte);
+ fault->pfn, iter->old_spte, fault->prefetch, true,
+ fault->map_writable, true, &new_spte);
if (new_spte == iter->old_spte)
ret = RET_PF_SPURIOUS;
@@ -1178,8 +1179,9 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
* Capture the dirty status of the page, so that it doesn't get
* lost when the SPTE is marked for access tracking.
*/
- if (is_writable_pte(iter->old_spte))
- kvm_set_pfn_dirty(spte_to_pfn(iter->old_spte));
+ if (is_writable_pte(iter->old_spte) &&
+ is_refcounted_page_spte(iter->old_spte))
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(iter->old_spte)));
new_spte = mark_spte_for_access_track(iter->old_spte);
iter->old_spte = kvm_tdp_mmu_write_spte(iter->sptep,
@@ -1602,7 +1604,8 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
trace_kvm_tdp_mmu_spte_changed(iter.as_id, iter.gfn, iter.level,
iter.old_spte,
iter.old_spte & ~dbit);
- kvm_set_pfn_dirty(spte_to_pfn(iter.old_spte));
+ if (is_refcounted_page_spte(iter.old_spte))
+ kvm_set_page_dirty(pfn_to_page(spte_to_pfn(iter.old_spte)));
}
rcu_read_unlock();
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index f72c79f159a2..cff5df6b0c52 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1211,6 +1211,9 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
void kvm_release_page_clean(struct page *page);
void kvm_release_page_dirty(struct page *page);
+void kvm_set_page_accessed(struct page *page);
+void kvm_set_page_dirty(struct page *page);
+
struct kvm_follow_pfn {
const struct kvm_memory_slot *slot;
gfn_t gfn;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5d66d841e775..e53a14adf149 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3281,17 +3281,19 @@ static bool kvm_is_ad_tracked_page(struct page *page)
return !PageReserved(page);
}
-static void kvm_set_page_dirty(struct page *page)
+void kvm_set_page_dirty(struct page *page)
{
if (kvm_is_ad_tracked_page(page))
SetPageDirty(page);
}
+EXPORT_SYMBOL_GPL(kvm_set_page_dirty);
-static void kvm_set_page_accessed(struct page *page)
+void kvm_set_page_accessed(struct page *page)
{
if (kvm_is_ad_tracked_page(page))
mark_page_accessed(page);
}
+EXPORT_SYMBOL_GPL(kvm_set_page_accessed);
void kvm_release_page_clean(struct page *page)
{