Re: [PATCH v10 031/108] KVM: x86/mmu: Replace hardcoded value 0 for the initial value for SPTE

Binbin Wu
2022-11-03T08:00:55Z

On 2022/11/3 15:17, Binbin Wu wrote:
>
> On 2022/10/30 14:22, isaku.yamahata@intel.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@intel.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@intel.com>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.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,