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

[Aili Yao <yaoaili126@gmail.com>]

On Tue, 23 Nov 2021 16:18:34 +0800
Aili Yao <yaoaili126@gmail.com> wrote:

> On Mon, 22 Nov 2021 19:13:02 +0000
> Sean Christopherson <seanjc@google.com> wrote:
> 
> > On Mon, Nov 22, 2021, Aili Yao wrote:
> > > From: Aili Yao <yaoaili@kingsoft.com>
> > > 
> > > 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@kingsoft.com>
> > > ---
> > >  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. 
> 
> I checked this, It seems we will set vcpu->cpu to a valid value when we
> create vcpu( kvm_vm_ioctl_create_vcpu()), 

Really Sorry, My code base is too old; This vcpu->cpu assignment has been deleted
in latest code, And this housekeeping_cpu() check will result problem.

Thanks!

>only after that we can
> configure lapic through vcpu fd and start the timer, this may not be one
> real problem.
> 
> Currently, the patch seems work as expected in my test, maybe one
> possible candidate for the issue listed above.
> 
> Thanks
> 
> > 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?
> > 
> > >  }
> > >  
> > >  bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> > > -- 
> > > 2.25.1
> > >   
>