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SubjectRe: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt
On Tue, 7 Dec 2021 23:23:03 +0000
Sean Christopherson <seanjc@google.com> wrote:

> On Wed, Nov 24, 2021, Aili Yao wrote:
> > When cpu-pm is successfully enabled, and hlt_in_guest is true and
> > mwait_in_guest is false, the guest cant't use Monitor/Mwait instruction
> > for idle operation, instead, the guest may use halt for that purpose, as
> > we have enable the cpu-pm feature and hlt_in_guest is true, we will also
> > minimize the guest exit; For such a scenario, Monitor/Mwait instruction
> > support is totally disabled, the guest has no way to use Mwait to exit from
> > non-root mode;
> >
> > For cpu-pm feature, hlt_in_guest and others except mwait_in_guest will
> > be a good hint for it. So replace it with hlt_in_guest.
>
> This should be a separate patch from the housekeeping_cpu() check, if we add
> the housekeeping check.
>
> > Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
> > ---
> > arch/x86/kvm/lapic.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 759952dd1222..42aef1accd6b 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -34,6 +34,7 @@
> > #include <asm/delay.h>
> > #include <linux/atomic.h>
> > #include <linux/jump_label.h>
> > +#include <linux/sched/isolation.h>
> > #include "kvm_cache_regs.h"
> > #include "irq.h"
> > #include "ioapic.h"
> > @@ -113,13 +114,14 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> > + !housekeeping_cpu(vcpu->cpu, HK_FLAG_TIMER);
>
> Why not check kvm_{hlt,mwait}_in_guest()? IIUC, non-housekeeping CPUs don't _have_
> to be associated 1:1 with a vCPU, in which case posting the timer is unlikely
> to be a performance win even though the target isn't a housekeeping CPU.

Yes, non-housekeeping CPUs can be assigned to multi vCPUs, I don't think it's a common configuration;
But this can happen.

> And wouldn't exposing HLT/MWAIT to a vCPU that's on a housekeeping CPU be a bogus
> configuration?

Agree, it's a bogus configuration and not suppose to like this, but this can happen;

It seems we can't cover all the abnormal cases in a single line. So I think just checking for
most right configurations is needed.

> > }
> >
> > bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> > {
> > return kvm_x86_ops.set_hv_timer
> > - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> > + && !(kvm_hlt_in_guest(vcpu->kvm) ||
>
> This is incorrect, the HLT vs. MWAIT isn't purely a posting interrupts thing. The
> VMX preemption timer counts down in C0, C1, and C2, but not deeper sleep states.
> HLT is always C1, thus it's safe to use the VMX preemption timer even if the guest
> can execute HLT without exiting.
> The timer isn't compatible with MWAIT because it stops counting in C3 (or lower),
> i.e. the guest can cause the timer to stop counting.

Thanks for your pointer, now i know this.

>
> > kvm_can_post_timer_interrupt(vcpu));
> > }
> > EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
> > --
>
> Splicing in Wanpeng's version to try and merge the two threads:
>
> On Tue, Nov 23, 2021 at 10:00 PM Wanpeng Li <kernellwp@gmail.com> wrote:
> > ---
> > arch/x86/kvm/lapic.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 759952dd1222..8257566d44c7 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -113,14 +113,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_mwait_in_guest(vcpu->kvm) && kvm_vcpu_apicv_active(vcpu);
>
> As Aili's changelog pointed out, MWAIT may not be advertised to the guest.
>
> So I think we want this? With a non-functional, opinionated refactoring of
> kvm_can_use_hv_timer() because I'm terrible at reading !(a || b).
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 40270d7bc597..c77cb386d03d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -113,14 +113,25 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
>
> static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> {
> - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> + (kvm_mwait_in_guest(vcpu) || kvm_hlt_in_guest(vcpu));
> }

I think only kvm_hlt_in_guest() check is enough here, as for current code, if kvm_mwait_in_guest() is true,
kvm_hlt_in_guest must be ture, if kvm_mwait_in_guest() is false, kvm_hlt_in_guest() could also
be true.

> bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> {
> - return kvm_x86_ops.set_hv_timer
> - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> - kvm_can_post_timer_interrupt(vcpu));
> + /*
> + * Don't use the hypervisor timer, a.k.a. VMX Preemption Timer, if the
> + * guest can execute MWAIT without exiting as the timer will stop
> + * counting if the core enters C3 or lower. HLT in the guest is ok as
> + * HLT is effectively C1 and the timer counts in C0, C1, and C2.
> + *
> + * Don't use the hypervisor timer if KVM can post a timer interrupt to
> + * the guest since posted the timer avoids taking an extra a VM-Exit
> + * when the timer expires.
> + */
> + return kvm_x86_ops.set_hv_timer &&
> + !kvm_mwait_in_guest(vcpu->kvm) &&
> + !kvm_can_post_timer_interrupt(vcpu));
> }
> EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
>

I think this modification covers most used configurations and it's right.
Thanks!

--Aili Yao


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