> -----邮件原件-----
> 发件人: Andy Lutomirski <luto@xxxxxxxxxx>
> 发送时间: 2021年10月5日 6:57
> 收件人: Li,Rongqing <lirongqing@xxxxxxxxx>; kvm@xxxxxxxxxxxxxxx;
> wanpengli@xxxxxxxxxxx; jan.kiszka@xxxxxxxxxxx; x86@xxxxxxxxxx
> 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@xxxxxxxxx>
> > ---
> > 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;
> > }
> >
> >
