Re: [PATCH 22/28] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

Ben Gardon
2021-11-22T23:00:32Z

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Don't flush the TLBs when zapping all TDP MMU pages, as the only time KVM
> uses the slow version of "zap everything" is when the VM is being
> destroyed or the owning mm has exited. In either case, KVM_RUN is
> unreachable for the VM, i.e. the guest TLB entries cannot be consumed.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 31fb622249e5..e5401f0efe8e 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -888,14 +888,15 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
>
> void kvm_tdp_mmu_zap_all(struct kvm *kvm)
> {
> - bool flush = false;
> int i;
>
> + /*
> + * A TLB flush is unnecessary, KVM's zap everything if and only the VM
> + * is being destroyed or the userspace VMM has exited. In both cases,
> + * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request.
> + */

Nit: Suggest:
/*
* A TLB flush is unnecessary. KVM's zap_all is used if and
only if the VM
* is being destroyed or the userspace VMM has exited. In both cases,
* the vCPUs are not running and will never run again, so their
TLB state doesn't matter.
*/

> for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> - flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, flush);
> -
> - if (flush)
> - kvm_flush_remote_tlbs(kvm);
> + (void)kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, false);
> }
>
> /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>