Re: [PATCH 07/23] KVM: MMU: remove kvm_mmu_calc_root_page_role

[Paolo Bonzini]

On 2/10/22 18:29, Sean Christopherson wrote:
> On Thu, Feb 10, 2022, Paolo Bonzini wrote:
> > On 2/10/22 01:47, Sean Christopherson wrote:
> > > The nested mess is likely easily solved, I don't see any obvious issue with swapping
> > > the order.  But I still don't love the subtlety.  I do like shaving cycles, just
> > > not the subtlety...
> >
> > Not so easily, but it's doable and it's essentially what I did in the other
> > series (the one that reworks the root cache).
>
> Sounds like I should reveiw that series first?

Yeah, this one is still a nice step in the direction of guest pt walk 
from shadow pt build(*), but with no immediate use for TDP MMU root reuse.

The original idea was to use the MMU role to decide whether to do 
kvm_mmu_unload(), but that would have still been a bandaid---inefficient 
and a bad idea overall.  Patches 6+7 of this series (once fixed, because 
they were buggy as hell) turned out to be enough to tame the PGD cache 
and remove kvm_mmu_unload() altogether from kvm_mmu_reset_context().

Paolo

(*) Your idea of detecting stale roots is quite easily implemented on 
top of these, for example, because a root is stale if and only if the 
root_role changes.