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: <3FEB4D87-EEAF-4A21-BCBC-291A4A7C2230@gmail.com>
Date: Fri, 12 Sep 2025 20:33:39 +0900
From: Itaru Kitayama <itaru.kitayama@...il.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Itaru Kitayama <itaru.kitayama@...ux.dev>,
 Oliver Upton <oliver.upton@...ux.dev>, Joey Gouly <joey.gouly@....com>,
 K Poulose Suzuki <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 Sep 12, 2025, at 20:01, Marc Zyngier <maz@...nel.org> wrote:
> 
> 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.
> 

Then I’ll drop this patch.

Thanks,
Itaru.

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