Re: [PATCH v4 2/2] KVM: x86/mmu: Consider the hva in mmu_notifier retry

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 22, 2021, David Stevens wrote:
> ---
> v3 -> v4:
>  - Skip prefetch while invalidations are in progress

Oof, nice catch.

...

> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 9ac0a727015d..f6aaac729667 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2758,6 +2758,13 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
>  	if (sp->role.level > PG_LEVEL_4K)
>  		return;
>  
> +	/*
> +	 * If addresses are being invalidated, skip prefetching to avoid
> +	 * accidentally prefetching those addresses.
> +	 */
> +	if (unlikely(vcpu->kvm->mmu_notifier_count))
> +		return;

FNAME(pte_prefetch) needs the same check.

Paolo, this brings up a good addition for the work to integrate the mmu notifier
into the rest of KVM, e.g. for vmcs12 pages.  Ideally, gfn_to_page_many_atomic()
and __gfn_to_pfn_memslot() would WARN if mmu_notifier_count is non-zero, but
that will fire all over the place until the nested code properly integrates the
notifier.  There are a few use cases where racing with the notifier is acceptable,
e.g. reexecute_instruction(), but hopefully we can address those flows without
things getting too ugly.

> +
>  	__direct_pte_prefetch(vcpu, sp, sptep);
>  }