Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

[Paolo Bonzini]

On 01/04/21 18:50, Ben Gardon wrote:
> > This is just cosmetic, but I'd prefer to keep the call to
> > kvm_tdp_mmu_invalidate_roots at the original place, so that it's clear
> > in the next patch that it's two separate parts because of the different
> > locking requirements.
> I'm not sure exactly what you mean and I could certainly do a better
> job explaining in the commit description, but it's actually quite
> important that kvm_tdp_mmu_invalidate_roots at least precede
> kvm_zap_obsolete_pages as kvm_zap_obsolete_pages drops the lock and
> yields. If kvm_tdp_mmu_invalidate_roots doesn't go first then vCPUs
> could wind up dropping their ref on an old root and then picking it up
> again before the last root had a chance to drop its ref.
> Explaining in the description that kvm_tdp_mmu_zap_all is being
> dropped because it is no longer necessary (as opposed to being moved)
> might help make that cleaner.

No, what would help is the remark you just made about 
kvm_zap_obsolete_pages yielding.  But that doesn't matter after 13/13 
though, does it?  Perhaps it's easier to just combine them.

Paolo