linux-kernel - Re: [PATCH 2/2] KVM: x86/mmu: Exclude the MMU

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Date:   Tue, 09 Mar 2021 15:31:21 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Exclude the MMU_PRESENT bit from MMIO
 SPTE's generation

On Tue, 2021-03-09 at 14:12 +0100, Paolo Bonzini wrote:
> 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.

This is exactly what I expected to happen, I just didn't find that code.
Thanks for explanation, and it shows that I didn't study the mmio spte
code much.

Best regards,
	Maxim Levitsky

> 
> Paolo
>