Re: [PATCH v2 07/17] KVM: x86/mmu: Check PDPTRs before allocating PAE roots

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

On 08/04/21 17:48, Sean Christopherson wrote:
> Freaking PDPTRs.  I was really hoping we could keep the lock and pages_available()
> logic outside of the helpers.  What if kvm_mmu_load() reads the PDPTRs and
> passes them into mmu_alloc_shadow_roots()?  Or is that too ugly?

The patch I have posted (though untested) tries to do that in a slightly 
less ugly way by pushing make_mmu_pages_available down to mmu_alloc_*_roots.

Paolo

> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index efb41f31e80a..e3c4938cd665 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3275,11 +3275,11 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>          return 0;
>   }
>
> -static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> +static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu, u64 pdptrs[4])
>   {
>          struct kvm_mmu *mmu = vcpu->arch.mmu;
> -       u64 pdptrs[4], pm_mask;
>          gfn_t root_gfn, root_pgd;
> +       u64 pm_mask;
>          hpa_t root;
>          int i;
>
> @@ -3291,11 +3291,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>
>          if (mmu->root_level == PT32E_ROOT_LEVEL) {
>                  for (i = 0; i < 4; ++i) {
> -                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
> -                       if (!(pdptrs[i] & PT_PRESENT_MASK))
> -                               continue;
> -
> -                       if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
> +                       if ((pdptrs[i] & PT_PRESENT_MASK) &&
> +                           mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
>                                  return 1;
>                  }
>          }
> @@ -4844,21 +4841,33 @@ EXPORT_SYMBOL_GPL(kvm_mmu_reset_context);
>
>   int kvm_mmu_load(struct kvm_vcpu *vcpu)
>   {
> -       int r;
> +       struct kvm_mmu *mmu = vcpu->arch.mmu;
> +       u64 pdptrs[4];
> +       int r, i;
>
> -       r = mmu_topup_memory_caches(vcpu, !vcpu->arch.mmu->direct_map);
> +       r = mmu_topup_memory_caches(vcpu, !mmu->direct_map);
>          if (r)
>                  goto out;
>          r = mmu_alloc_special_roots(vcpu);
>          if (r)
>                  goto out;
> +
> +       /*
> +        * On SVM, reading PDPTRs might access guest memory, which might fault
> +        * and thus might sleep.  Grab the PDPTRs before acquiring mmu_lock.
> +        */
> +       if (!mmu->direct_map && mmu->root_level == PT32E_ROOT_LEVEL) {
> +               for (i = 0; i < 4; ++i)
> +                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
> +       }
> +
>          write_lock(&vcpu->kvm->mmu_lock);
>          if (make_mmu_pages_available(vcpu))
>                  r = -ENOSPC;
>          else if (vcpu->arch.mmu->direct_map)
>                  r = mmu_alloc_direct_roots(vcpu);
>          else
> -               r = mmu_alloc_shadow_roots(vcpu);
> +               r = mmu_alloc_shadow_roots(vcpu, pdptrs);
>          write_unlock(&vcpu->kvm->mmu_lock);
>          if (r)
>                  goto out;