Re: [PATCH 1/1] KVM: arm64: PMU: Fix PMCR_EL0 reset value

[Ricardo Koller]

On Fri, Dec 09, 2022 at 05:58:31PM +0000, Oliver Upton wrote:
> On Fri, Dec 09, 2022 at 04:44:46PM +0000, James Clark wrote:
> > ARMV8_PMU_PMCR_N_MASK is an unshifted value which results in the wrong
> > reset value for PMCR_EL0, so shift it to fix it.
> 
> That's just mean. *_MASK tends to be a shifted mask, although it would
> appear that asm/perf_event.h does not follow this convention. Fixing
> that would be nice (as I'm sure somebody else will get burned by this),
> but for the sake of an immediate fix:
>

Even kvm-unit-tests does this:

arm/pmu.c:
#define PMU_PMCR_N_SHIFT   11
#define PMU_PMCR_N_MASK    0x1f


> > This fixes the following error when running qemu:
> > 
> >   $ qemu-system-aarch64 -cpu host -machine type=virt,accel=kvm -kernel ...
> > 
> >   target/arm/helper.c:1813: pmevcntr_rawwrite: Assertion `counter < pmu_num_counters(env)' failed.
> > 
> > Fixes: 292e8f149476 ("KVM: arm64: PMU: Simplify PMCR_EL0 reset handling")
> > Signed-off-by: James Clark <james.clark@xxxxxxx>
> 
> Reviewed-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> 
> --
> Thanks,
> Oliver
> 
> > ---
> >  arch/arm64/kvm/sys_regs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index d5ee52d6bf73..c6cbfe6b854b 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -646,7 +646,7 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> >  		return;
> >  
> >  	/* Only preserve PMCR_EL0.N, and reset the rest to 0 */
> > -	pmcr = read_sysreg(pmcr_el0) & ARMV8_PMU_PMCR_N_MASK;
> > +	pmcr = read_sysreg(pmcr_el0) & (ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
> >  	if (!kvm_supports_32bit_el0())
> >  		pmcr |= ARMV8_PMU_PMCR_LC;
> >  
> > -- 
> > 2.25.1
> > 
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