Re: [PATCH v11 030/113] KVM: x86/mmu: Replace hardcoded value 0 for the initial value for SPTE

[Zhi Wang]

On Thu, 12 Jan 2023 08:31:38 -0800
isaku.yamahata@xxxxxxxxx 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@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         | 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,