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Date: Thu, 3 Nov 2022 16:00:44 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: isaku.yamahata@...el.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: 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 v10 031/108] KVM: x86/mmu: Replace hardcoded value 0 for
the initial value for SPTE
On 2022/11/3 15:17, Binbin Wu wrote:
>
> On 2022/10/30 14:22, isaku.yamahata@...el.com wrote:
>> 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 doesn't use this bit; 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/spte.h | 2 ++
>> arch/x86/kvm/mmu/tdp_mmu.c | 15 ++++++------
>> 3 files changed, 54 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 10017a9f26ee..e7e11f51f8b4 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -538,9 +538,9 @@ 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;
>> @@ -574,7 +574,7 @@ 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)
>> @@ -642,6 +642,39 @@ static void walk_shadow_page_lockless_end(struct
>> kvm_vcpu *vcpu)
>> }
>> }
>
> Why the changes in mmu_spte_clear_track_bits and
> mmu_spte_clear_no_track are not handled differently for X86_64 and non
> X86_64 as the shadow page init below?
> It seems the two functions are the common code.
I see there are different definitions of SHADOW_NONPRESENT_VALUE for
X86_64 and non X86_64 in the following patch.
So got the answer now. : )
>
>
>> +#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;
>> @@ -651,8 +684,7 @@ 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) {
>> @@ -5870,7 +5902,13 @@ 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/spte.h b/arch/x86/kvm/mmu/spte.h
>> index 7670c13ce251..42ecaa75da15 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 eab765442d0b..38bc4c2f0f1f 100644
>> --- a/arch/x86/kvm/mmu/tdp_mmu.c
>> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
>> @@ -694,7 +694,7 @@ 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;
>> }
>> @@ -871,8 +871,8 @@ 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;
>> }
>> }
>> @@ -928,8 +928,9 @@ 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;
>> }
>> @@ -963,7 +964,7 @@ 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;
>> }
>> @@ -1328,7 +1329,7 @@ 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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