linux-kernel - Re: [PATCH] PMCR_EL0.N is RAZ/WI. At least a build failes in Ubuntu 22.04 LTS. Remove the set function.

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Message-ID: <867by4c4v1.wl-maz@kernel.org>
Date: Fri, 12 Sep 2025 12:00:50 +0100
From: Marc Zyngier <maz@...nel.org>
To: Itaru Kitayama <itaru.kitayama@...ux.dev>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev,
	kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Itaru Kitayama <itaru.kitayama@...itsu.com>
Subject: Re: [PATCH] PMCR_EL0.N is RAZ/WI. At least a build failes in Ubuntu 22.04 LTS. Remove the set function.

On Fri, 12 Sep 2025 09:27:40 +0100,
Itaru Kitayama <itaru.kitayama@...ux.dev> wrote:
> 
> Signed-off-by: Itaru Kitayama <itaru.kitayama@...itsu.com>

This isn't an acceptable commit message.

> ---
> Seen a build failure with old Ubuntu 22.04 LTS, while the latest release
> has no build issue, a write to the bit fields is RAZ/WI, remove the
> function.
> ---
>  tools/testing/selftests/kvm/arm64/vpmu_counter_access.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/arm64/vpmu_counter_access.c b/tools/testing/selftests/kvm/arm64/vpmu_counter_access.c
> index f16b3b27e32ed7ca57481f27d689d47783aa0345..56214a4430be90b3e1d840f2719b22dd44f0b49b 100644
> --- a/tools/testing/selftests/kvm/arm64/vpmu_counter_access.c
> +++ b/tools/testing/selftests/kvm/arm64/vpmu_counter_access.c
> @@ -45,11 +45,6 @@ static uint64_t get_pmcr_n(uint64_t pmcr)
>  	return FIELD_GET(ARMV8_PMU_PMCR_N, pmcr);
>  }
>  
> -static void set_pmcr_n(uint64_t *pmcr, uint64_t pmcr_n)
> -{
> -	u64p_replace_bits((__u64 *) pmcr, pmcr_n, ARMV8_PMU_PMCR_N);
> -}
> -
>  static uint64_t get_counters_mask(uint64_t n)
>  {
>  	uint64_t mask = BIT(ARMV8_PMU_CYCLE_IDX);
> @@ -490,7 +485,6 @@ static void test_create_vpmu_vm_with_pmcr_n(uint64_t pmcr_n, bool expect_fail)
>  	 * Setting a larger value of PMCR.N should not modify the field, and
>  	 * return a success.
>  	 */
> -	set_pmcr_n(&pmcr, pmcr_n);
>  	vcpu_set_reg(vcpu, KVM_ARM64_SYS_REG(SYS_PMCR_EL0), pmcr);
>  	pmcr = vcpu_get_reg(vcpu, KVM_ARM64_SYS_REG(SYS_PMCR_EL0));
>  
> 

So what are you fixing here? A build failure? A semantic defect?
Something else? What makes this a valid change?

Frankly, I have no idea.

But KVM definitely allows PMCR_EL0.N to be written from userspace, and
that's not going to change.

	M.

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