linux-kernel - Re: [PATCH 1/1] KVM: arm64: PMU: Fix PMCR

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Date:   Fri, 9 Dec 2022 17:58:31 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     James Clark <james.clark@....com>
Cc:     kvmarm@...ts.linux.dev, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] KVM: arm64: PMU: Fix PMCR_EL0 reset value

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:

> 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@....com>

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>

--
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
>