Re: [PATCH] KVM: Clear pv eoi pending bit only when it is set

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Li RongQing <lirongqing@baidu.com> writes:

> clear pv eoi pending bit only when it is set, to avoid calling
> pv_eoi_put_user()
>
> and this can speed pv_eoi_clr_pending about 300 nsec on AMD EPYC
> most of the time
>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  arch/x86/kvm/lapic.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 76fb009..c434f70 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -694,9 +694,9 @@ static void pv_eoi_set_pending(struct kvm_vcpu *vcpu)
>  	__set_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention);
>  }
>  
> -static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
> +static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu, bool pending)

Nitpick (and probably a matter of personal taste): pv_eoi_clr_pending()
has only one user and the change doesn't make its interface much nicer,
I'd suggest we just inline in instead. (we can probably do the same to
pv_eoi_get_pending()/pv_eoi_set_pending() too).

>  {
> -	if (pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0) {
> +	if (pending && pv_eoi_put_user(vcpu, KVM_PV_EOI_DISABLED) < 0) {
>  		printk(KERN_WARNING "Can't clear EOI MSR value: 0x%llx\n",
>  			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
>  		return;
> @@ -2693,7 +2693,8 @@ static void apic_sync_pv_eoi_from_guest(struct kvm_vcpu *vcpu,
>  	 * While this might not be ideal from performance point of view,
>  	 * this makes sure pv eoi is only enabled when we know it's safe.
>  	 */
> -	pv_eoi_clr_pending(vcpu);
> +	pv_eoi_clr_pending(vcpu, pending);
> +
>  	if (pending)
>  		return;
>  	vector = apic_set_eoi(apic);

Could you probably elaborate a bit (probably by enhancing the comment
above pv_eoi_clr_pending()) why the race we have here (even before the
patch) doesn't matter? As far as I understand it, the guest can change
PV EOI status from a different CPU (it shouldn't do it but it still can)
at any time: e.g. between pv_eoi_get_pending() and pv_eoi_clr_pending()
but also right after we do pv_eoi_clr_pending() so the patch doesn't
really change much in this regard.

-- 
Vitaly