Re: [PATCH 03/21] KVM: x86/mmu: Replace hardcoded value 0 for the initial value for SPTE

[Binbin Wu <binbin.wu@linux.intel.com>]

On 2/28/2024 7:20 AM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@google.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 <seanjc@google.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Message-Id: <acdf09bf60cad12c495005bf3495c54f6b3069c9.1705965635.git.isaku.yamahata@intel.com>
> [Remove unnecessary CONFIG_X86_64 check. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   arch/x86/kvm/mmu/mmu.c         | 14 +++++++++-----
>   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, 19 insertions(+), 13 deletions(-)

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

BTW, does it prefer to add "No functional change intended." in changelog for
a patch like this?

>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e4cc7f764980..b5baf11359ad 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -567,9 +567,9 @@ 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;
> @@ -603,7 +603,7 @@ 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)
> @@ -1950,7 +1950,8 @@ 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)
>   		return 0;
>   
>   	return vcpu->arch.mmu->sync_spte(vcpu, sp, i);
> @@ -6173,7 +6174,10 @@ 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;
> +	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 4d4e98fe4f35..bebd73cd61bb 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -911,7 +911,7 @@ 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 a129951c9a88..4d1799ba2bf8 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -149,6 +149,8 @@ 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 d078157e62aa..c8a4d92497b4 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -603,7 +603,7 @@ 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;
>   }
> @@ -740,8 +740,8 @@ 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;
>   	}
>   }
> @@ -808,8 +808,8 @@ 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;
>   }
> @@ -843,7 +843,7 @@ 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);
>   
>   		/*
>   		 * Zappings SPTEs in invalid roots doesn't require a TLB flush,
> @@ -1276,7 +1276,7 @@ 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->arg.pte)) {
>   		new_spte = kvm_mmu_changed_pte_notifier_make_spte(iter->old_spte,