* 答复: [PATCH] KVM: x86: directly call wbinvd for local cpu when emulate wbinvd
2021-10-04 22:56 ` Andy Lutomirski
@ 2021-10-05 8:04 ` Li,Rongqing
2021-10-12 8:48 ` Li,Rongqing
1 sibling, 0 replies; 4+ messages in thread
From: Li,Rongqing @ 2021-10-05 8:04 UTC (permalink / raw)
To: Andy Lutomirski, kvm, wanpengli, jan.kiszka, x86
> -----邮件原件-----
> 发件人: Andy Lutomirski <luto@kernel.org>
> 发送时间: 2021年10月5日 6:57
> 收件人: Li,Rongqing <lirongqing@baidu.com>; kvm@vger.kernel.org;
> wanpengli@tencent.com; jan.kiszka@siemens.com; x86@kernel.org
> 主题: Re: [PATCH] KVM: x86: directly call wbinvd for local cpu when emulate
> wbinvd
>
> On 9/28/21 02:27, Li RongQing wrote:
> > directly call wbinvd for local pCPU, which can avoid ipi for itself
> > and calling of get_cpu/on_each_cpu_mask/etc.
> >
>
> Why is this an improvement? Trading get_cpu() vs preempt_disable() seems
> like a negligible difference, and it makes the code more complicated.
>
First: to local pCpu, this reduces a ipi to itself, ipi will trigger context switch between irq and thread, it is expensive.
Second, preempt_disable/preempt_enable vs get_cpu/ cpumask_set_cpu/put_cpu, the preempt_disable/ preempt_enable is more slight.
And this can avoid the atomic cpumask_set_cpu
-Li
> > In fact, This change reverts commit 2eec73437487 ("KVM: x86: Avoid
> > issuing wbinvd twice"), since smp_call_function_many is skiping the
> > local cpu (as description of c2162e13d6e2f), wbinvd is not issued
> > twice
> >
> > and reverts commit c2162e13d6e2f ("KVM: X86: Fix missing local pCPU
> > when executing wbinvd on all dirty pCPUs") too, which fixed the
> > previous patch, when revert previous patch, it is not needed.
> >
> > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > ---
> > arch/x86/kvm/x86.c | 13 ++++++-------
> > 1 files changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> > 28ef141..ee65941 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6984,15 +6984,14 @@ static int kvm_emulate_wbinvd_noskip(struct
> kvm_vcpu *vcpu)
> > return X86EMUL_CONTINUE;
> >
> > if (static_call(kvm_x86_has_wbinvd_exit)()) {
> > - int cpu = get_cpu();
> > -
> > - cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);
> > - on_each_cpu_mask(vcpu->arch.wbinvd_dirty_mask,
> > + preempt_disable();
> > + smp_call_function_many(vcpu->arch.wbinvd_dirty_mask,
> > wbinvd_ipi, NULL, 1);
> > - put_cpu();
> > + preempt_enable();
> > cpumask_clear(vcpu->arch.wbinvd_dirty_mask);
> > - } else
> > - wbinvd();
> > + }
> > +
> > + wbinvd();
> > return X86EMUL_CONTINUE;
> > }
> >
> >
^ permalink raw reply [flat|nested] 4+ messages in thread* 答复: [PATCH] KVM: x86: directly call wbinvd for local cpu when emulate wbinvd
2021-10-04 22:56 ` Andy Lutomirski
2021-10-05 8:04 ` 答复: " Li,Rongqing
@ 2021-10-12 8:48 ` Li,Rongqing
1 sibling, 0 replies; 4+ messages in thread
From: Li,Rongqing @ 2021-10-12 8:48 UTC (permalink / raw)
To: Andy Lutomirski, kvm, wanpengli, jan.kiszka, x86
> -----邮件原件-----
> 发件人: Li,Rongqing
> 发送时间: 2021年10月5日 16:05
> 收件人: 'Andy Lutomirski' <luto@kernel.org>; kvm@vger.kernel.org;
> wanpengli@tencent.com; jan.kiszka@siemens.com; x86@kernel.org
> 主题: 答复: [PATCH] KVM: x86: directly call wbinvd for local cpu when emulate
> wbinvd
>
>
>
> > -----邮件原件-----
> > 发件人: Andy Lutomirski <luto@kernel.org>
> > 发送时间: 2021年10月5日 6:57
> > 收件人: Li,Rongqing <lirongqing@baidu.com>; kvm@vger.kernel.org;
> > wanpengli@tencent.com; jan.kiszka@siemens.com; x86@kernel.org
> > 主题: Re: [PATCH] KVM: x86: directly call wbinvd for local cpu when
> > emulate wbinvd
> >
> > On 9/28/21 02:27, Li RongQing wrote:
> > > directly call wbinvd for local pCPU, which can avoid ipi for itself
> > > and calling of get_cpu/on_each_cpu_mask/etc.
> > >
> >
> > Why is this an improvement? Trading get_cpu() vs preempt_disable()
> > seems like a negligible difference, and it makes the code more complicated.
> >
>
> First: to local pCpu, this reduces a ipi to itself, ipi will trigger context switch
> between irq and thread, it is expensive.
>
on_each_cpu_mask will not send ipi to local cpu, the callback function will be executed directly for local cpu
But this patch is useful still, Using smp_call_function_many and directly calling wbinvd can reduce the unnecessary cpumask_set_cpu and cpumask_test_cpu(), and some duplicate preempt disable
-Li
> Second, preempt_disable/preempt_enable vs get_cpu/
> cpumask_set_cpu/put_cpu, the preempt_disable/ preempt_enable is more
> slight.
>
> And this can avoid the atomic cpumask_set_cpu
>
> -Li
>
>
> > > In fact, This change reverts commit 2eec73437487 ("KVM: x86: Avoid
> > > issuing wbinvd twice"), since smp_call_function_many is skiping the
> > > local cpu (as description of c2162e13d6e2f), wbinvd is not issued
> > > twice
> > >
> > > and reverts commit c2162e13d6e2f ("KVM: X86: Fix missing local pCPU
> > > when executing wbinvd on all dirty pCPUs") too, which fixed the
> > > previous patch, when revert previous patch, it is not needed.
> > >
> > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > ---
> > > arch/x86/kvm/x86.c | 13 ++++++-------
> > > 1 files changed, 6 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> > > 28ef141..ee65941 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -6984,15 +6984,14 @@ static int kvm_emulate_wbinvd_noskip(struct
> > kvm_vcpu *vcpu)
> > > return X86EMUL_CONTINUE;
> > >
> > > if (static_call(kvm_x86_has_wbinvd_exit)()) {
> > > - int cpu = get_cpu();
> > > -
> > > - cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);
> > > - on_each_cpu_mask(vcpu->arch.wbinvd_dirty_mask,
> > > + preempt_disable();
> > > + smp_call_function_many(vcpu->arch.wbinvd_dirty_mask,
> > > wbinvd_ipi, NULL, 1);
> > > - put_cpu();
> > > + preempt_enable();
> > > cpumask_clear(vcpu->arch.wbinvd_dirty_mask);
> > > - } else
> > > - wbinvd();
> > > + }
> > > +
> > > + wbinvd();
> > > return X86EMUL_CONTINUE;
> > > }
> > >
> > >
^ permalink raw reply [flat|nested] 4+ messages in thread