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Date: Fri, 23 Feb 2024 11:00:37 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: David Stevens <stevensd@...omium.org>,
Sean Christopherson <seanjc@...gle.com>
Cc: Yu Zhang <yu.c.zhang@...ux.intel.com>,
Isaku Yamahata <isaku.yamahata@...il.com>,
Zhi Wang <zhi.wang.linux@...il.com>, Maxim Levitsky <mlevitsk@...hat.com>,
kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v10 7/8] KVM: x86/mmu: Track if sptes refer to refcounted
pages
On 2/21/24 08:25, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
>
> 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@...omium.org>
> ---
> 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)
> {
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