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Date: Wed, 25 Jan 2023 11:24:34 +0200
From: Zhi Wang <zhi.wang.linux@...il.com>
To: isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
Sagi Shahar <sagis@...gle.com>,
David Matlack <dmatlack@...gle.com>,
Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v11 030/113] KVM: x86/mmu: Replace hardcoded value 0 for
the initial value for SPTE
On Thu, 12 Jan 2023 08:31:38 -0800
isaku.yamahata@...el.com wrote:
This refactor patch is quite hacky.
Why not change the purpose of vcpu->arch.mmu_shadow_page.gfp_zero and let the
callers respect that the initial value of spte can be configurable? It will be
generic and not TDX-specific, then kvm_init_shadow_page() is not required,
mmu_topup_shadow_page_cache() can be left un-touched as the refactor can cover
other architectures.
1) Let it store the expected nonpresent value and rename it to nonpresent_spte.
2) Let mmu_spte_clear_track_bits(), mmu_spte_clear_no_track() and all
the other places where assume 0 as initial value, respect nonpreset_spte.
3) Let kvm_mmu_topup_memory_cache() to respect nonpresent_spte: a. using GFP_ZERO
if the nonpresent_spte is zero. b. memset the page if nonpresent_spte is *not*
zero.
Now the initial value is configurable, configure the nonpresent_spte in the TDX
initialization path before the first topup in the next patch.
> From: Isaku Yamahata <isaku.yamahata@...el.com>
>
> The TDX support will need the "suppress #VE" bit (bit 63) set as the
> initial value for SPTE. To reduce code change size, introduce a new macro
> SHADOW_NONPRESENT_VALUE for the initial value for the shadow page table
> entry (SPTE) and replace hard-coded value 0 for it. Initialize shadow page
> tables with their value.
>
> The plan is to unconditionally set the "suppress #VE" bit for both AMD and
> Intel as: 1) AMD hardware uses the bit 63 as NX for present SPTE and
> ignored for non-present SPTE; 2) for conventional VMX guests, KVM never
> enables the "EPT-violation #VE" in VMCS control and "suppress #VE" bit is
> ignored by hardware.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 50 ++++++++++++++++++++++++++++++----
> arch/x86/kvm/mmu/paging_tmpl.h | 3 +-
> arch/x86/kvm/mmu/spte.h | 2 ++
> arch/x86/kvm/mmu/tdp_mmu.c | 15 +++++-----
> 4 files changed, 56 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 15d0e8f11d53..59befdfeec23 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -540,9 +540,9 @@ static u64 mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
>
> if (!is_shadow_present_pte(old_spte) ||
> !spte_has_volatile_bits(old_spte))
> - __update_clear_spte_fast(sptep, 0ull);
> + __update_clear_spte_fast(sptep, SHADOW_NONPRESENT_VALUE);
> else
> - old_spte = __update_clear_spte_slow(sptep, 0ull);
> + old_spte = __update_clear_spte_slow(sptep, SHADOW_NONPRESENT_VALUE);
>
> if (!is_shadow_present_pte(old_spte))
> return old_spte;
> @@ -576,7 +576,7 @@ static u64 mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
> */
> static void mmu_spte_clear_no_track(u64 *sptep)
> {
> - __update_clear_spte_fast(sptep, 0ull);
> + __update_clear_spte_fast(sptep, SHADOW_NONPRESENT_VALUE);
> }
>
> static u64 mmu_spte_get_lockless(u64 *sptep)
> @@ -644,6 +644,39 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
> }
> }
>
> +#ifdef CONFIG_X86_64
> +static inline void kvm_init_shadow_page(void *page)
> +{
> + memset64(page, SHADOW_NONPRESENT_VALUE, 4096 / 8);
> +}
> +
> +static int mmu_topup_shadow_page_cache(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_mmu_memory_cache *mc = &vcpu->arch.mmu_shadow_page_cache;
> + int start, end, i, r;
> +
> + start = kvm_mmu_memory_cache_nr_free_objects(mc);
> + r = kvm_mmu_topup_memory_cache(mc, PT64_ROOT_MAX_LEVEL);
> +
> + /*
> + * Note, topup may have allocated objects even if it failed to allocate
> + * the minimum number of objects required to make forward progress _at
> + * this time_. Initialize newly allocated objects even on failure, as
> + * userspace can free memory and rerun the vCPU in response to -ENOMEM.
> + */
> + end = kvm_mmu_memory_cache_nr_free_objects(mc);
> + for (i = start; i < end; i++)
> + kvm_init_shadow_page(mc->objects[i]);
> + return r;
> +}
> +#else
> +static int mmu_topup_shadow_page_cache(struct kvm_vcpu *vcpu)
> +{
> + return kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> + PT64_ROOT_MAX_LEVEL);
> +}
> +#endif /* CONFIG_X86_64 */
> +
> static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> {
> int r;
> @@ -653,8 +686,7 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> 1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
> if (r)
> return r;
> - r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> - PT64_ROOT_MAX_LEVEL);
> + r = mmu_topup_shadow_page_cache(vcpu);
> if (r)
> return r;
> if (maybe_indirect) {
> @@ -5920,7 +5952,13 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
> vcpu->arch.mmu_page_header_cache.kmem_cache = mmu_page_header_cache;
> vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
>
> - vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> + /*
> + * When X86_64, initial SEPT entries are initialized with
> + * SHADOW_NONPRESENT_VALUE. Otherwise zeroed. See
> + * mmu_topup_shadow_page_cache().
> + */
> + if (!IS_ENABLED(CONFIG_X86_64))
> + vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
>
> vcpu->arch.mmu = &vcpu->arch.root_mmu;
> vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 0f6455072055..42d7106c7350 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -1036,7 +1036,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> gpa_t pte_gpa;
> gfn_t gfn;
>
> - if (!sp->spt[i])
> + /* spt[i] has initial value of shadow page table allocation */
> + if (sp->spt[i] == SHADOW_NONPRESENT_VALUE)
> continue;
>
> pte_gpa = first_pte_gpa + i * sizeof(pt_element_t);
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 0d8deefee66c..f190eaf6b2b5 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -148,6 +148,8 @@ static_assert(MMIO_SPTE_GEN_LOW_BITS == 8 && MMIO_SPTE_GEN_HIGH_BITS == 11);
>
> #define MMIO_SPTE_GEN_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_BITS + MMIO_SPTE_GEN_HIGH_BITS - 1, 0)
>
> +#define SHADOW_NONPRESENT_VALUE 0ULL
> +
> extern u64 __read_mostly shadow_host_writable_mask;
> extern u64 __read_mostly shadow_mmu_writable_mask;
> extern u64 __read_mostly shadow_nx_mask;
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 12e430a4ebc3..9cf5844dd34a 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -701,7 +701,7 @@ static inline int tdp_mmu_zap_spte_atomic(struct kvm *kvm,
> * 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, SHADOW_NONPRESENT_VALUE);
>
> return 0;
> }
> @@ -878,8 +878,8 @@ static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> continue;
>
> if (!shared)
> - tdp_mmu_set_spte(kvm, &iter, 0);
> - else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0))
> + tdp_mmu_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> + else if (tdp_mmu_set_spte_atomic(kvm, &iter, SHADOW_NONPRESENT_VALUE))
> goto retry;
> }
> }
> @@ -935,8 +935,9 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> if (WARN_ON_ONCE(!is_shadow_present_pte(old_spte)))
> return false;
>
> - __tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte, 0,
> - sp->gfn, sp->role.level + 1, true, true);
> + __tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte,
> + SHADOW_NONPRESENT_VALUE, sp->gfn, sp->role.level + 1,
> + true, true);
>
> return true;
> }
> @@ -970,7 +971,7 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> !is_last_spte(iter.old_spte, iter.level))
> continue;
>
> - tdp_mmu_set_spte(kvm, &iter, 0);
> + tdp_mmu_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> flush = true;
> }
>
> @@ -1339,7 +1340,7 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
> * invariant that the PFN of a present * leaf SPTE can never change.
> * See __handle_changed_spte().
> */
> - tdp_mmu_set_spte(kvm, iter, 0);
> + tdp_mmu_set_spte(kvm, iter, SHADOW_NONPRESENT_VALUE);
>
> if (!pte_write(range->pte)) {
> new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,
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