Re: [PATCH] KVM: x86/mmu: simplify kvm_tdp_mmu_map flow when guest has to retry

[David Matlack]

On Thu, Nov 17, 2022 at 8:14 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> A removed SPTE is never present, hence the "if" in kvm_tdp_mmu_map
> only fails in the exact same conditions that the earlier loop
> tested in order to issue a  "break". So, instead of checking twice the
> condition (upper level SPTEs could not be created or was frozen), just
> exit the loop with a goto---the usual poor-man C replacement for RAII
> early returns.
>
> While at it, do not use the "ret" variable for return values of
> functions that do not return a RET_PF_* enum.  This is clearer
> and also makes it possible to initialize ret to RET_PF_RETRY.
>
> Suggested-by: Robert Hoo <robert.hu@xxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 40 ++++++++++++++++++--------------------
>  1 file changed, 19 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index e08596775427..771210ce5181 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1159,7 +1159,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>         struct kvm *kvm = vcpu->kvm;
>         struct tdp_iter iter;
>         struct kvm_mmu_page *sp;
> -       int ret;
> +       int ret = RET_PF_RETRY;
>
>         kvm_mmu_hugepage_adjust(vcpu, fault);
>
> @@ -1168,23 +1168,25 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>         rcu_read_lock();
>
>         tdp_mmu_for_each_pte(iter, mmu, fault->gfn, fault->gfn + 1) {
> +               int r;
> +
>                 if (fault->nx_huge_page_workaround_enabled)
>                         disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
>
>                 if (iter.level == fault->goal_level)
>                         break;
>
> -               /* Step down into the lower level page table if it exists. */
> -               if (is_shadow_present_pte(iter.old_spte) &&
> -                   !is_large_pte(iter.old_spte))
> -                       continue;
> -
>                 /*
>                  * If SPTE has been frozen by another thread, just give up and
>                  * retry, avoiding unnecessary page table allocation and free.
>                  */
>                 if (is_removed_spte(iter.old_spte))
> -                       break;
> +                       goto retry;
> +
> +               /* Step down into the lower level page table if it exists. */
> +               if (is_shadow_present_pte(iter.old_spte) &&
> +                   !is_large_pte(iter.old_spte))
> +                       continue;
>
>                 /*
>                  * The SPTE is either non-present or points to a huge page that
> @@ -1196,13 +1198,17 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>                 sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
>
>                 if (is_shadow_present_pte(iter.old_spte))
> -                       ret = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
> +                       r = tdp_mmu_split_huge_page(kvm, &iter, sp, true);
>                 else
> -                       ret = tdp_mmu_link_sp(kvm, &iter, sp, true);
> +                       r = tdp_mmu_link_sp(kvm, &iter, sp, true);

Can this fix be squashed into [1]? It seems like a serious enough bug.
If 2 threads race to update the same PTE, KVM will return -EBUSY out
to userspace from KVM_RUN, I think. I'm not sure about QEMU, but that
would be fatal for the VM in Vanadium.

[1] https://lore.kernel.org/kvm/20221109185905.486172-3-dmatlack@xxxxxxxxxx/

>
> -               if (ret) {
> +               /*
> +                * Also force the guest to retry the access if the upper level SPTEs
> +                * aren't in place.
> +                */
> +               if (r) {
>                         tdp_mmu_free_sp(sp);
> -                       break;
> +                       goto retry;
>                 }
>
>                 if (fault->huge_page_disallowed &&
> @@ -1213,18 +1219,10 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>                 }
>         }
>
> -       /*
> -        * Force the guest to retry the access if the upper level SPTEs aren't
> -        * in place, or if the target leaf SPTE is frozen by another CPU.
> -        */
> -       if (iter.level != fault->goal_level || is_removed_spte(iter.old_spte)) {
> -               rcu_read_unlock();
> -               return RET_PF_RETRY;
> -       }
> -
>         ret = tdp_mmu_map_handle_target_level(vcpu, fault, &iter);
> -       rcu_read_unlock();
>
> +retry:
> +       rcu_read_unlock();
>         return ret;
>  }
>
> --
> 2.31.1
>