Re: [PATCH v14 031/113] KVM: x86/mmu: Replace hardcoded value 0 for the initial value for SPTE

[Yuan Yao]

On Sun, May 28, 2023 at 09:19:13PM -0700, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c         | 20 +++++++++++++++-----
>  arch/x86/kvm/mmu/paging_tmpl.h |  2 +-
>  arch/x86/kvm/mmu/spte.h        |  2 ++
>  arch/x86/kvm/mmu/tdp_mmu.c     | 14 +++++++-------
>  4 files changed, 25 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index dc2b9a2f717c..1b6fd4434e96 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -576,9 +576,9 @@ 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;
> @@ -612,7 +612,7 @@ 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)
> @@ -1969,7 +1969,8 @@ static bool kvm_sync_page_check(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>
>  static int kvm_sync_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int i)
>  {
> -	if (!sp->spt[i])
> +	/* sp->spt[i] has initial value of shadow page table allocation */
> +	if (sp->spt[i] != SHADOW_NONPRESENT_VALUE)

This should be "sp->spt[i] == SHADOW_NONPRESENT_VALUE" ? Looks all present spt[i]
are skipped without sync for shadow paging.

>  		return 0;
>
>  	return vcpu->arch.mmu->sync_spte(vcpu, sp, i);
> @@ -6120,7 +6121,16 @@ 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_memory_cache_alloc_obj().
> +	 */
> +	if (IS_ENABLED(CONFIG_X86_64))
> +		vcpu->arch.mmu_shadow_page_cache.init_value =
> +			SHADOW_NONPRESENT_VALUE;
> +	if (!vcpu->arch.mmu_shadow_page_cache.init_value)
> +		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 0662e0278e70..ef8124bd2f11 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -892,7 +892,7 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
>  	gpa_t pte_gpa;
>  	gfn_t gfn;
>
> -	if (WARN_ON_ONCE(!sp->spt[i]))
> +	if (WARN_ON_ONCE(sp->spt[i] == SHADOW_NONPRESENT_VALUE))
>  		return 0;
>
>  	first_pte_gpa = FNAME(get_level1_sp_gpa)(sp);
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 1279db2eab44..a99eb7d4ae5d 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 3000ef6d79ea..ddd995885dd3 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -627,7 +627,7 @@ 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;
>  }
> @@ -764,8 +764,8 @@ static void __tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
>  			continue;
>
>  		if (!shared)
> -			tdp_mmu_iter_set_spte(kvm, &iter, 0);
> -		else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0))
> +			tdp_mmu_iter_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> +		else if (tdp_mmu_set_spte_atomic(kvm, &iter, SHADOW_NONPRESENT_VALUE))
>  			goto retry;
>  	}
>  }
> @@ -821,8 +821,8 @@ 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);
> +	tdp_mmu_set_spte(kvm, kvm_mmu_page_as_id(sp), sp->ptep, old_spte,
> +			 SHADOW_NONPRESENT_VALUE, sp->gfn, sp->role.level + 1);
>
>  	return true;
>  }
> @@ -856,7 +856,7 @@ 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_iter_set_spte(kvm, &iter, 0);
> +		tdp_mmu_iter_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
>  		flush = true;
>  	}
>
> @@ -1250,7 +1250,7 @@ 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_iter_set_spte(kvm, iter, 0);
> +	tdp_mmu_iter_set_spte(kvm, iter, SHADOW_NONPRESENT_VALUE);
>
>  	if (!pte_write(range->pte)) {
>  		new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,
> --
> 2.25.1
>