Re: [PATCH v6 08/11] KVM: x86: Optimize kvm_{test_,}age_gfn a little bit

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 25, 2024, David Matlack wrote:
> On 2024-07-24 01:10 AM, James Houghton wrote:
> > Optimize both kvm_age_gfn and kvm_test_age_gfn's interaction with the
> 
> nit: Use () when referring to functions.
> 
> > shadow MMU by, rather than checking if our memslot has rmaps, check if
> > there are any indirect_shadow_pages at all.
> 
> What is optimized by checking indirect_shadow_pages instead of
> have_rmaps and what's the benefit? Smells like a premature optimization.

Checking indirect_shadow_pages avoids taking mmu_lock for write when KVM doesn't
currently have shadow MMU pages, but did at some point in the past, whereas
kvm_memslots_have_rmaps() is sticky and will return true forever.

> > Also, for kvm_test_age_gfn, reorder the TDP MMU check to be first. If we
> > find that the range is young, we do not need to check the shadow MMU.
> 
> This should be a separate commit since it's a logically distinct change
> and no dependency on the other change in this commit (other than both
> touch the same function).
> 
> Splitting the commits up will also make it easier to write more specific
> short logs (instead of "optimize a little bit" :)

+1.  Especially code movement and refactoring, e.g. factoring out
tdp_mmu_clear_spte_bits_atomic() would ideally be in a standalone patch that's
dead simple to review.