2022-02-18 09:02:20

by Wanpeng Li

Subject: [PATCH] x86/kvm: Don't use pv tlb/ipi/sched_yield if on 1 vCPU

From: Wanpeng Li <[email protected]>

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 <[email protected]>
---
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 a438217cbfac..f734e3b0cfec 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -462,19 +462,22 @@ 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)
--
2.25.1

2022-02-18 09:05:45

by Paolo Bonzini

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

On 2/18/22 09:10, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> 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 <[email protected]>
> ---
> 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 a438217cbfac..f734e3b0cfec 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -462,19 +462,22 @@ 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)

Queued, thanks.

Paolo