Re: [PATCH 2/2] KVM: x86/mmu: Exclude the MMU_PRESENT bit from MMIO SPTE's generation

[Paolo Bonzini <pbonzini@redhat.com>]

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