Re: [PATCH v3 01/12] Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"

[Sean Christopherson]

On Tue, Aug 03, 2021, Paolo Bonzini wrote:
> On 02/08/21 20:33, Maxim Levitsky wrote:
> > From: Sean Christopherson <seanjc@xxxxxxxxxx>
> > 
> > This together with the next patch will fix a future race between
> > kvm_zap_gfn_range and the page fault handler, which will happen
> > when AVIC memslot is going to be only partially disabled.
> > 
> > This is based on a patch suggested by Sean Christopherson:
> > https://lkml.org/lkml/2021/7/22/1025
> 
> I'll also add a small note from the original message:
> 
>     The performance impact is minimal since kvm_zap_gfn_range is only called by
>     users, update_mtrr() and kvm_post_set_cr0().  Both only use it if the guest
>     has non-coherent DMA, in order to honor the guest's UC memtype.  MTRR and CD
>     setup only happens at boot, and generally in an area where the page tables
>     should be small (for CD) or should not include the affected GFNs at all
>     (for MTRRs).
> 
> On top of this, I think the CD case (kvm_post_set_cr0) can be changed to use
> kvm_mmu_zap_all_fast.

No, because fast zap requires kvm->slots_lock be held.  That could be relaxed by
reverting ca333add6933 ("KVM: x86/mmu: Explicitly track only a single invalid mmu
generation") and converting mmu_valid_gen to a u64 (to prevent wrap on 32-bit KVM).
IMO the extra memory cost, even though it's meager savings when using TDP without
nested, isn't worth relaxing the rules for fast zap.  Non-coherent DMA isn't very
common these days, and toggling CR0.CD is a rare guest operation (it'd probably
never happen if the darn architcture didn't set it on RESET).