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

[Paolo Bonzini]

On 03/08/21 17:11, Sean Christopherson wrote:
> > 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.

Yeah, that much I knew. :)  The issue is the usual ordering between 
slots_lock and SRCU.

Paolo

  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).