[PATCH v8 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow_pfn
Mika Penttilä
mika.penttila at mbosol.com
Thu Aug 24 19:13:42 AEST 2023
On 8/24/23 11:04, David Stevens wrote:
> From: David Stevens <stevensd at chromium.org>
>
> Stop passing FOLL_GET to __kvm_follow_pfn. This allows the host to map
> memory into the guest that is backed by un-refcounted struct pages - for
> example, the tail pages of higher order non-compound pages allocated by
> the amdgpu driver via ttm_pool_alloc_page.
>
> The bulk of this change is tracking the is_refcounted_page flag so that
> non-refcounted pages don't trigger page_count() == 0 warnings. This is
> done by storing the flag in an unused bit in the sptes. This only bit is
> not available in PAE SPTEs, so FOLL_GET is only omitted for TDP on
> x86-64.
>
> Signed-off-by: David Stevens <stevensd at chromium.org>
> ---
> arch/x86/kvm/mmu/mmu.c | 55 +++++++++++++++++++++++----------
> arch/x86/kvm/mmu/mmu_internal.h | 1 +
> arch/x86/kvm/mmu/paging_tmpl.h | 8 +++--
> arch/x86/kvm/mmu/spte.c | 4 ++-
> arch/x86/kvm/mmu/spte.h | 12 ++++++-
> arch/x86/kvm/mmu/tdp_mmu.c | 22 +++++++------
> include/linux/kvm_host.h | 3 ++
> virt/kvm/kvm_main.c | 6 ++--
> 8 files changed, 79 insertions(+), 32 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index dabae67f198b..4f5d33e95c6e 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -553,12 +553,14 @@ 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_pte(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_pte(old_spte))
> + kvm_set_page_dirty(pfn_to_page(spte_to_pfn(old_spte)));
> }
>
> return flush;
> @@ -596,14 +598,18 @@ static u64 mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
> * 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(page && !page_count(page));
> + if (is_refcounted_page_pte(old_spte)) {
> + page = kvm_pfn_to_refcounted_page(pfn);
> + WARN_ON(!page || !page_count(page));
> + }
>
> - if (is_accessed_spte(old_spte))
> - kvm_set_pfn_accessed(pfn);
> + if (is_refcounted_page_pte(old_spte)) {
> + 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;
> }
> @@ -639,8 +645,8 @@ 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_pte(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);
> @@ -1278,8 +1284,8 @@ 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_pte(*sptep))
> + kvm_set_page_dirty(pfn_to_page(spte_to_pfn(*sptep)));
>
> return was_writable;
> }
> @@ -2937,6 +2943,11 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> bool host_writable = !fault || fault->map_writable;
> bool prefetch = !fault || fault->prefetch;
> bool write_fault = fault && fault->write;
> + /*
> + * Prefetching uses gfn_to_page_many_atomic, which never gets
> + * non-refcounted pages.
> + */
> + bool is_refcounted = !fault || fault->is_refcounted_page;
>
> pgprintk("%s: spte %llx write_fault %d gfn %llx\n", __func__,
> *sptep, write_fault, gfn);
> @@ -2969,7 +2980,7 @@ 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, is_refcounted, &spte);
>
> if (*sptep == spte) {
> ret = RET_PF_SPURIOUS;
> @@ -4296,11 +4307,19 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
> static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> {
> struct kvm_memory_slot *slot = fault->slot;
> + /*
> + * We only allow non-refcounted pages if we can track whether or not
> + * pages are refcounted via an SPTE bit. This bit is not available
> + * in PAE SPTEs, so pass FOLL_GET if we may have to deal with those.
> + */
> + unsigned int get_flag = (tdp_enabled && IS_ENABLED(CONFIG_X86_64)) ?
> + 0 : FOLL_GET;
> struct kvm_follow_pfn foll = {
> .slot = slot,
> .gfn = fault->gfn,
> - .flags = FOLL_GET | (fault->write ? FOLL_WRITE : 0),
> + .flags = (fault->write ? FOLL_WRITE : 0) | get_flag,
> .try_map_writable = true,
> + .guarded_by_mmu_notifier = true,
> };
>
> /*
> @@ -4317,6 +4336,7 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> fault->slot = NULL;
> fault->pfn = KVM_PFN_NOSLOT;
> fault->map_writable = false;
> + fault->is_refcounted_page = false;
> return RET_PF_CONTINUE;
> }
> /*
> @@ -4372,6 +4392,7 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> success:
> fault->hva = foll.hva;
> fault->map_writable = foll.writable;
> + fault->is_refcounted_page = foll.is_refcounted_page;
> return RET_PF_CONTINUE;
> }
>
> @@ -4456,8 +4477,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> r = direct_map(vcpu, fault);
>
> out_unlock:
> + if (fault->is_refcounted_page)
> + kvm_set_page_accessed(pfn_to_page(fault->pfn));
> write_unlock(&vcpu->kvm->mmu_lock);
> - kvm_release_pfn_clean(fault->pfn);
Should we check FOLL_GET is actually omitted to skip the kvm_release_pfn_clean() ?
> return r;
> }
>
> @@ -4534,8 +4556,9 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
> r = kvm_tdp_mmu_map(vcpu, fault);
>
> out_unlock:
> + if (fault->is_refcounted_page)
> + kvm_set_page_accessed(pfn_to_page(fault->pfn));
> read_unlock(&vcpu->kvm->mmu_lock);
> - kvm_release_pfn_clean(fault->pfn);
> return r;
> }
> #endif
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index d39af5639ce9..55790085884f 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -240,6 +240,7 @@ struct kvm_page_fault {
> kvm_pfn_t pfn;
> hva_t hva;
> bool map_writable;
> + bool is_refcounted_page;
>
> /*
> * Indicates the guest is trying to write a gfn that contains one or
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 0662e0278e70..4ffcebc0c3ce 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -828,8 +828,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> r = FNAME(fetch)(vcpu, fault, &walker);
>
> out_unlock:
> + if (fault->is_refcounted_page)
> + kvm_set_page_accessed(pfn_to_page(fault->pfn));
> write_unlock(&vcpu->kvm->mmu_lock);
> - kvm_release_pfn_clean(fault->pfn);
> return r;
> }
>
> @@ -883,7 +884,7 @@ 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;
> @@ -940,10 +941,11 @@ 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 = spte & SPTE_MMU_PAGE_REFCOUNTED;
> 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 cf2c6426a6fc..5b13d9143d56 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 (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 1279db2eab44..be93dd061ae3 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -95,6 +95,11 @@ 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:
> @@ -332,6 +337,11 @@ static inline bool is_dirty_spte(u64 spte)
> return dirty_mask ? spte & dirty_mask : spte & PT_WRITABLE_MASK;
> }
>
> +static inline bool is_refcounted_page_pte(u64 spte)
> +{
> + return spte & SPTE_MMU_PAGE_REFCOUNTED;
> +}
> +
> static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,
> int level)
> {
> @@ -462,7 +472,7 @@ 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 512163d52194..a9b1b14d2e26 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -474,6 +474,7 @@ 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_pte(old_spte);
>
> WARN_ON(level > PT64_ROOT_MAX_LEVEL);
> WARN_ON(level < PG_LEVEL_4K);
> @@ -538,9 +539,9 @@ 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
> @@ -552,9 +553,9 @@ 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)));
> }
>
> /*
> @@ -988,8 +989,9 @@ 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, fault->is_refcounted_page,
> + &new_spte);
>
> if (new_spte == iter->old_spte)
> ret = RET_PF_SPURIOUS;
> @@ -1205,8 +1207,9 @@ 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_pte(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,
> @@ -1626,7 +1629,8 @@ 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_pte(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 713fc2d91f95..292701339198 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1157,6 +1157,9 @@ 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 963b96cd8ff9..fa1848c6c84f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2949,17 +2949,19 @@ 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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