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Subject: Re: [PATCH 2/2] KVM: x86/mmu: Exclude the MMU_PRESENT bit from MMIO
 SPTE's generation
To:     Maxim Levitsky <mlevitsk@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Tom Lendacky <thomas.lendacky@amd.com>
References: <20210309021900.1001843-1-seanjc@google.com>
 <20210309021900.1001843-3-seanjc@google.com>
 <785c17c307e66b9d7b422cc577499d284cfb6e7b.camel@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d72993d9-c11c-a5f4-0452-b2bed730938c@redhat.com>
Date:   Tue, 9 Mar 2021 14:12:26 +0100
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On 09/03/21 11:09, Maxim Levitsky wrote:
> What happens if mmio generation overflows (e.g if userspace keeps on updating the memslots)?
> In theory if we have a SPTE with a stale generation, it can became valid, no?
> 
> I think that we should in the case of the overflow zap all mmio sptes.
> What do you think?

Zapping all MMIO SPTEs is done by updating the generation count.  When 
it overflows, all SPs are zapped:

         /*
          * The very rare case: if the MMIO generation number has wrapped,
          * zap all shadow pages.
          */
         if (unlikely(gen == 0)) {
                 kvm_debug_ratelimited("kvm: zapping shadow pages for 
mmio generation wraparound\n");
                 kvm_mmu_zap_all_fast(kvm);
         }

So giving it more bits make this more rare, at the same time having to 
remove one or two bits is not the end of the world.

Paolo