Re: [PATCH AUTOSEL 5.15 10/23] x86/kvm: Don't use pv tlb/ipi/sched_yield if on 1 vCPU

Paolo Bonzini
2022-03-01T20:19:44Z

On 3/1/22 21:16, Sasha Levin wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> [ Upstream commit ec756e40e271866f951d77c5e923d8deb6002b15 ]
>
> Inspired by commit 3553ae5690a (x86/kvm: Don't use pvqspinlock code if
> only 1 vCPU), on a VM with only 1 vCPU, there is no need to enable
> pv tlb/ipi/sched_yield and we can save the memory for __pv_cpu_mask.
>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> Message-Id: <1645171838-2855-1-git-send-email-wanpengli@tencent.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> arch/x86/kernel/kvm.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index b656456c3a944..811c7aaf23aac 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -457,19 +457,22 @@ static bool pv_tlb_flush_supported(void)
> {
> return (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
> !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> - kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
> + kvm_para_has_feature(KVM_FEATURE_STEAL_TIME) &&
> + (num_possible_cpus() != 1));
> }
>
> static bool pv_ipi_supported(void)
> {
> - return kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI);
> + return (kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI) &&
> + (num_possible_cpus() != 1));
> }
>
> static bool pv_sched_yield_supported(void)
> {
> return (kvm_para_has_feature(KVM_FEATURE_PV_SCHED_YIELD) &&
> !kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> - kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
> + kvm_para_has_feature(KVM_FEATURE_STEAL_TIME) &&
> + (num_possible_cpus() != 1));
> }
>
> #define KVM_IPI_CLUSTER_SIZE (2 * BITS_PER_LONG)

NACK

Not really necessary.

Paolo