Re: [PATCH v6 09/11] KVM: x86: Implement fast_only versions of kvm_{test_,}age_gfn

[David Matlack <dmatlack@google.com>]

On 2024-07-24 01:10 AM, James Houghton wrote:
> These fast-only versions simply ignore the shadow MMU. We can locklessly
> handle the shadow MMU later.
> 
> Set HAVE_KVM_MMU_NOTIFIER_YOUNG_FAST_ONLY for X86_64 only, as that is
> the only case where the TDP MMU might be used. Without the TDP MMU, the
> fast-only notifiers will always be no-ops. It would be ideal not to
> report has_fast_only if !tdp_mmu_enabled, but tdp_mmu_enabled can be
> changed at any time.

tdp_mmu_enabled is a read-only KVM parameter. And even when it was
writable, it was still fixed for a given VM at VM creation time.

Would it make more sense to have kvm_arch_post_init_vm() set
has_fast_aging if the architecture supports it. And for x86 that means
iff tdp_mmu_enabled.

> 
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>  arch/x86/kvm/Kconfig   | 1 +
>  arch/x86/kvm/mmu/mmu.c | 4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 6ac43074c5e9..ed9049cf1255 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -24,6 +24,7 @@ config KVM
>  	select KVM_COMMON
>  	select KVM_GENERIC_MMU_NOTIFIER
>  	select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
> +	select HAVE_KVM_MMU_NOTIFIER_YOUNG_FAST_ONLY if X86_64
>  	select HAVE_KVM_IRQCHIP
>  	select HAVE_KVM_PFNCACHE
>  	select HAVE_KVM_DIRTY_RING_TSO
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 919d59385f89..3c6c9442434a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1641,7 +1641,7 @@ bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  	if (tdp_mmu_enabled)
>  		young |= kvm_tdp_mmu_age_gfn_range(kvm, range);
>  
> -	if (kvm_has_shadow_mmu_sptes(kvm)) {
> +	if (!range->arg.fast_only && 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);
> @@ -1657,7 +1657,7 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  	if (tdp_mmu_enabled)
>  		young |= kvm_tdp_mmu_test_age_gfn(kvm, range);
>  
> -	if (!young && kvm_has_shadow_mmu_sptes(kvm)) {
> +	if (!young && !range->arg.fast_only && kvm_has_shadow_mmu_sptes(kvm)) {
>  		write_lock(&kvm->mmu_lock);
>  		young = kvm_handle_gfn_range(kvm, range, kvm_test_age_rmap);
>  		write_unlock(&kvm->mmu_lock);
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
>