Re: [PATCH v1] KVM: arm64: PMU: Restore the guest's EL0 event counting after migration

[Reiji Watanabe <reijiw@google.com>]

Hi Marc,

On Tue, Mar 28, 2023 at 12:08:31PM +0100, Marc Zyngier wrote:
> On Tue, 28 Mar 2023 04:47:25 +0100,
> Reiji Watanabe <reijiw@google.com> wrote:
> > 
> > Currently, with VHE, KVM enables the EL0 event counting for the
> > guest on vcpu_load() or KVM enables it as a part of the PMU
> > register emulation process, when needed.  However, in the migration
> > case (with VHE), the same handling is lacking.  So, enable it on the
> > first KVM_RUN with VHE (after the migration) when needed.
> 
> It wasn't completely clear to me how the migration case was affected
> by this until I started digging into the call stack:
> 
> At load-time, the PMCR_EL0 effects haven't been propagated yet (the
> events haven't been created, as this is what kvm_pmu_handle_pmcr()
> does on first run). So there is an ordering inversion between
> kvm_pmu_handle_pmcr() and kvm_vcpu_pmu_restore_guest().
> 
> Moving the latter call into the former fixes the issue, completely
> emulating an extra write to PMCR_EL0.
> 
> I think it would be worth capturing some of the above in the commit
> message so that it doesn't get lost...

I agree with that. I will add the explanation in the commit message,
and will post v2.

> 
> > 
> > Fixes: d0c94c49792c ("KVM: arm64: Restore PMU configuration on first run")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > ---
> >  arch/arm64/kvm/pmu-emul.c | 1 +
> >  arch/arm64/kvm/sys_regs.c | 1 -
> >  2 files changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index c243b10f3e15..5eca0cdd961d 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -558,6 +558,7 @@ void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
> >  		for_each_set_bit(i, &mask, 32)
> >  			kvm_pmu_set_pmc_value(kvm_vcpu_idx_to_pmc(vcpu, i), 0, true);
> >  	}
> > +	kvm_vcpu_pmu_restore_guest(vcpu);
> >  }
> >  
> >  static bool kvm_pmu_counter_is_enabled(struct kvm_pmc *pmc)
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 1b2c161120be..34688918c811 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -794,7 +794,6 @@ static bool access_pmcr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> >  		if (!kvm_supports_32bit_el0())
> >  			val |= ARMV8_PMU_PMCR_LC;
> >  		kvm_pmu_handle_pmcr(vcpu, val);
> > -		kvm_vcpu_pmu_restore_guest(vcpu);
> >  	} else {
> >  		/* PMCR.P & PMCR.C are RAZ */
> >  		val = __vcpu_sys_reg(vcpu, PMCR_EL0)
> 
> With the nitpicking above addressed, and should this go into 6.3 as a
> fix:
> 
> Reviewed-by: Marc Zyngier <maz@kernel.org>

Thank you!
Reiji


> 
> I can otherwise take it into 6.4, depending on what Oliver decides to
> do.
> 
> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.