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

[David Matlack]

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.

> 
> 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" :)

Also, the commit re-orders kvm_age_gfn() as well but the commit message
only mentions kvm_test_age_gfn(). No objection to keeping the two
functions consistent but it should be called out in the commit message.

> 
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7b93ce8f0680..919d59385f89 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1629,19 +1629,24 @@ static void rmap_add(struct kvm_vcpu *vcpu, const struct kvm_memory_slot *slot,
>  	__rmap_add(vcpu->kvm, cache, slot, spte, gfn, access);
>  }
>  
> +static bool kvm_has_shadow_mmu_sptes(struct kvm *kvm)
> +{
> +	return !tdp_mmu_enabled || READ_ONCE(kvm->arch.indirect_shadow_pages);
> +}
> +
>  bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	bool young = false;
>  
> -	if (kvm_memslots_have_rmaps(kvm)) {
> +	if (tdp_mmu_enabled)
> +		young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
> +
> +	if (kvm_has_shadow_mmu_sptes(kvm)) {
>  		write_lock(&kvm->mmu_lock);
>  		young = kvm_handle_gfn_range(kvm, range, kvm_age_rmap);
>  		write_unlock(&kvm->mmu_lock);
>  	}
>  
> -	if (tdp_mmu_enabled)
> -		young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
> -
>  	return young;
>  }
>  
> @@ -1649,15 +1654,15 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	bool young = false;
>  
> -	if (kvm_memslots_have_rmaps(kvm)) {
> +	if (tdp_mmu_enabled)
> +		young |= kvm_tdp_mmu_test_age_gfn(kvm, range);
> +
> +	if (!young && kvm_has_shadow_mmu_sptes(kvm)) {

nit: A short comment here might be helpful to explain why young is
checked.

>  		write_lock(&kvm->mmu_lock);
>  		young = kvm_handle_gfn_range(kvm, range, kvm_test_age_rmap);
>  		write_unlock(&kvm->mmu_lock);
>  	}
>  
> -	if (tdp_mmu_enabled)
> -		young |= kvm_tdp_mmu_test_age_gfn(kvm, range);
> -
>  	return young;
>  }
>  
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
>