Re: [PATCH 2/4] KVM: arm64: Reset last_steal on vCPU pid change

[Marc Zyngier <maz@kernel.org>]

On Mon, 17 Aug 2026 22:29:22 +0100,
Dongli Zhang <dongli.zhang@oracle.com> wrote:
> 
> 
> 
> On Mon, Aug 17, 2026 1:42:20AM -0700, Marc Zyngier wrote:
> > On Sun, 16 Aug 2026 06:33:03 +0100,
> > Dongli Zhang <dongli.zhang@oracle.com> wrote:

[...]

> >> diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
> >> index 4ceabaa4c30b..000bf49cc0fd 100644
> >> --- a/arch/arm64/kvm/pvtime.c
> >> +++ b/arch/arm64/kvm/pvtime.c
> >> @@ -32,6 +32,11 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
> >>  	srcu_read_unlock(&kvm->srcu, idx);
> >>  }
> >>  
> >> +void kvm_reset_stolen_time(struct kvm_vcpu *vcpu)
> >> +{
> >> +	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
> >> +}
> >> +
> >>  long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu)
> >>  {
> >>  	u32 feature = smccc_get_arg1(vcpu);
> > 
> > Why isn't this common code? I really don't see the point in making
> > this arch-specific code. I'd expect something like this (untested):
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index ace5801a592f..3fb77360af01 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -950,6 +950,8 @@ struct kvm_vcpu_arch {
> >  	pid_t pid;
> >  };
> >  
> > +#define kvm_arch_vcpu_last_steal(v)	(v)->arch.steal.last_steal
> > +
> >  /*
> >   * Each 'flag' is composed of a comma-separated triplet:
> >   *
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 45e784462ec6..117aeb49231a 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4459,6 +4459,9 @@ static long kvm_vcpu_ioctl(struct file *filp,
> >  			if (r)
> >  				break;
> >  
> > +			if (IS_ENABLED(CONFIG_HAVE_PV_STEAL_CLOCK_GEN))
> > +				kvm_arch_vcpu_last_steal(vcpu) = current->sched_info.run_delay;
> > +
> >  			newpid = get_task_pid(current, PIDTYPE_PID);
> >  			write_lock(&vcpu->pid_lock);
> >  			vcpu->pid = newpid;
> > 
> > where each architecture that implements steal time provides an
> > accessor, and the core code is in charge of the adjustment.
> > 
> > It also makes sure that we don't leave any architecture behind.
> > 
> Or how about making it something like below?
> 
> if (IS_ENABLED(CONFIG_HAVE_PV_STEAL_CLOCK_GEN))
> 	kvm_arch_vcpu_reset_last_steal(vcpu);
>
> That would still move the policy to common KVM code, while leaving the exact
> arch state to the architecture implementation.

I don't think architectures should have a say in this. Steal time, as
a PV service, should not involve the architectures at all. After all,
that's the whole point of a PV service: hypervisor-specific hacks that
do not fit in the architectural envelope.

Bonus points if you move the last_steal field in the main vcpu
structure instead of some arch-specific one.

> For x86, the hook can reset vcpu->arch.st.last_steal for regular KVM steal time.
> If we also decide to cover Xen runstate in this series, the same x86 hook can
> additionally reset vcpu->arch.xen.last_steal.

For arch-specific stuff, there is the pid change hook already.

	M.

-- 
Without deviation from the norm, progress is not possible.