Re: [PATCH v3 14/28] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

Mingwei Zhang
2022-03-03T18:51:09Z

On Sat, Feb 26, 2022, Sean Christopherson 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>

Reviewed-by: Mingwei Zhang <mizhang@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 c231b60e1726..87706e9cc6f3 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -874,14 +874,15 @@ 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 zaps 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.
> + */
> 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);
> }
>
> /*
> --
> 2.35.1.574.g5d30c73bfb-goog
>