Re: [PATCH 1/2] kvm: mmu/x86: Remove redundant spte present check in mmu_set_spte

Sean Christopherson
2021-07-21T16:27:28Z

On Tue, Jul 20, 2021, Mingwei Zhang wrote:
> Drop an unnecessary is_shadow_present_pte() check when updating the rmaps
> after installing a non-MMIO SPTE. set_spte() is used only to create
> shadow-present SPTEs, e.g. MMIO SPTEs are handled early on, mmu_set_spte()
> runs with mmu_lock held for write, i.e. the SPTE can't be zapped between
> writing the SPTE and updating the rmaps.
>
> Opportunistically combine the "new SPTE" logic for large pages and rmaps.

Heh, except you forgot to actually do this in the code.

> No functional change intended.
>
> Suggested-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b888385d1933..c45ddd2c964f 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2693,12 +2693,10 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> if (!was_rmapped && is_large_pte(*sptep))
> ++vcpu->kvm->stat.lpages;
>
> - if (is_shadow_present_pte(*sptep)) {
> - if (!was_rmapped) {
> - rmap_count = rmap_add(vcpu, sptep, gfn);
> - if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> - rmap_recycle(vcpu, sptep, gfn);
> - }
> + if (!was_rmapped) {

As above, this should be:

if (!was_rmapped) {
if (is_large_pte(*sptep))
++vcpu->kvm->stat.lpages;

> + rmap_count = rmap_add(vcpu, sptep, gfn);
> + if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> + rmap_recycle(vcpu, sptep, gfn);
> }
>
> return ret;
> --
> 2.32.0.402.g57bb445576-goog
>