linux-kernel - Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge

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Date:   Tue, 9 Aug 2022 16:48:11 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 5/8] KVM: x86/mmu: Set disallowed_nx_huge_page in TDP
 MMU before setting SPTE

On 8/9/22 16:44, Sean Christopherson wrote:
> On Tue, Aug 09, 2022, Paolo Bonzini wrote:
>> On 8/9/22 05:26, Yan Zhao wrote:
>>> hi Sean,
>>>
>>> I understand this smp_rmb() is intended to prevent the reading of
>>> p->nx_huge_page_disallowed from happening before it's set to true in
>>> kvm_tdp_mmu_map(). Is this understanding right?
>>>
>>> If it's true, then do we also need the smp_rmb() for read of sp->gfn in
>>> handle_removed_pt()? (or maybe for other fields in sp in other places?)
>>
>> No, in that case the barrier is provided by rcu_dereference().  In fact, I
>> am not sure the barriers are needed in this patch either (but the comments
>> are :)):
> 
> Yeah, I'm 99% certain the barriers aren't strictly required, but I didn't love the
> idea of depending on other implementation details for the barriers.  Of course I
> completely overlooked the fact that all other sp fields would need the same
> barriers...
> 
>> - the write barrier is certainly not needed because it is implicit in
>> tdp_mmu_set_spte_atomic's cmpxchg64
>>
>> - the read barrier _should_ also be provided by rcu_dereference(pt), but I'm
>> not 100% sure about that. The reasoning is that you have
>>
>> (1)	iter->old spte = READ_ONCE(*rcu_dereference(iter->sptep));
>> 	...
>> (2)	tdp_ptep_t pt = spte_to_child_pt(old_spte, level);
>> (3)	struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
>> 	...
>> (4)	if (sp->nx_huge_page_disallowed) {
>>
>> and (4) is definitely ordered after (1) thanks to the READ_ONCE hidden
>> within (3) and the data dependency from old_spte to sp.
> 
> Yes, I think that's correct.  Callers must verify the SPTE is present before getting
> the associated child shadow page.  KVM does have instances where a shadow page is
> retrieved from the SPTE _pointer_, but that's the parent shadow page, i.e. isn't
> guarded by the SPTE being present.
> 
> 	struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(iter->sptep));
> 
> Something like this is as a separate patch?

Would you resubmit without the memory barriers then?

> diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> index f0af385c56e0..9d982ccf4567 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.h
> +++ b/arch/x86/kvm/mmu/tdp_iter.h
> @@ -13,6 +13,12 @@
>    * to be zapped while holding mmu_lock for read, and to allow TLB flushes to be
>    * batched without having to collect the list of zapped SPs.  Flows that can
>    * remove SPs must service pending TLB flushes prior to dropping RCU protection.
> + *
> + * The READ_ONCE() ensures that, if the SPTE points at a child shadow page, all
> + * fields in struct kvm_mmu_page will be read after the caller observes the
> + * present SPTE (KVM must check that the SPTE is present before following the
> + * SPTE's pfn to its associated shadow page).  Pairs with the implicit memory

I guess you mean both the shadow page table itself and the struct 
kvm_mmu_page?  Or do you think to_shadow_page() should have a smp_rmb()?

Paolo