Re: [PATCH] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt

From: Wanpeng Li
Date: Mon Nov 22 2021 - 21:57:56 EST


On Tue, 23 Nov 2021 at 03:14, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Mon, Nov 22, 2021, Aili Yao wrote:
> > From: Aili Yao <yaoaili@xxxxxxxxxxxx>
> >
> > When we isolate some pyhiscal cores, We may not use them for kvm guests,
> > We may use them for other purposes like DPDK, or we can make some kvm
> > guests isolated and some not, the global judgement pi_inject_timer is
> > not enough; We may make wrong decisions:
> >
> > In such a scenario, the guests without isolated cores will not be
> > permitted to use vmx preemption timer, and tscdeadline fastpath also be
> > disabled, both will lead to performance penalty.
> >
> > So check whether the vcpu->cpu is isolated, if not, don't post timer
> > interrupt.
> >
> > Signed-off-by: Aili Yao <yaoaili@xxxxxxxxxxxx>
> > ---
> > arch/x86/kvm/lapic.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 759952dd1222..72dde5532101 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"
> > @@ -113,7 +114,8 @@ 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);
>
> I don't think this is safe, vcpu->cpu will be -1 if the vCPU isn't scheduled in.
> This also doesn't play nice with the admin forcing pi_inject_timer=1. Not saying
> there's a reasonable use case for doing that, but it's supported today and this
> would break that behavior. It would also lead to weird behavior if a vCPU were
> migrated on/off a housekeeping vCPU. Again, probably not a reasonable use case,
> but I don't see anything that would outright prevent that behavior.
>
> The existing behavior also feels a bit unsafe as pi_inject_timer is writable while
> KVM is running, though I supposed that's orthogonal to this discussion.
>
> Rather than check vcpu->cpu, is there an existing vCPU flag that can be queried,
> e.g. KVM_HINTS_REALTIME?

How about something like below: