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

[Itaru Kitayama <itaru.kitayama@linux.dev>]

> On Sep 12, 2025, at 21:11, Marc Zyngier <maz@kernel.org> wrote:
> 
> On Fri, 12 Sep 2025 12:33:39 +0100,
> Itaru Kitayama <itaru.kitayama@gmail.com> wrote:
>> 
>> 
>> 
>>> On Sep 12, 2025, at 20:01, Marc Zyngier <maz@kernel.org> wrote:
>>> 
>>> On Fri, 12 Sep 2025 09:27:40 +0100,
>>> Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
>>>> 
>>>> Signed-off-by: Itaru Kitayama <itaru.kitayama@fujitsu.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.
> 
> I'm not asking you to drop it, I'm asking you to explain. If you found
> a problem, let's discuss it and fix it. But as it stands, you're not
> giving me much to go on.
> 

You are right, while the bit fields are write ignored, to be consistent with the handling of other bit fields of the register, I’m fully convinced that checking the write operation in the vpmu_counter_access.c file should be kept.

The build error I’ve seen with Ubuntu 22.04 LTS is below: 

gcc -D_GNU_SOURCE=  -I/home/itaru/projects/linux/tools/testing/selftests/cgroup/lib/include -DDEBUG -Wall -Wstrict-prototypes -Wuninitialized -O0 -g -std=gnu99 -Wno-gnu-variable-sized-type-not-at-end -MD -MP -DCONFIG_64BIT -fno-builtin-memcmp -fno-builtin-memcpy -fno-builtin-memset -fno-builtin-strnlen -fno-stack-protector -fno-PIE -fno-strict-aliasing -I/home/itaru/projects/linux/tools/testing/selftests/../../../tools/include -I/home/itaru/projects/linux/tools/testing/selftests/../../../tools/arch/arm64/include -I/home/itaru/projects/linux/tools/testing/selftests/../../../usr/include/ -Iinclude -Iarm64 -Iinclude/arm64 -I ../rseq -I..  -isystem /home/itaru/projects/linux/tools/testing/selftests/../../../usr/include -I/home/itaru/projects/linux/tools/testing/selftests/../../../tools/arch/arm64/include/generated/   -c arm64/vpmu_counter_access.c -o /home/itaru/projects/linux/tools/testing/selftests/kvm/arm64/vpmu_counter_access.o
In file included from /home/itaru/projects/linux/tools/testing/selftests/../../../tools/arch/arm64/include/asm/sysreg.h:1098,
                 from /home/itaru/projects/linux/tools/testing/selftests/../../../tools/arch/arm64/include/asm/esr.h:10,
                 from include/arm64/processor.h:16,
                 from arm64/vpmu_counter_access.c:16:
In function ‘field_multiplier’,
    inlined from ‘field_mask’ at /home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:141:17,
    inlined from ‘u64_encode_bits’ at /home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:172:1,
    inlined from ‘u64p_replace_bits’ at /home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:172:1,
    inlined from ‘set_pmcr_n’ at arm64/vpmu_counter_access.c:50:2:
/home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:136:17: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
  136 |                 __bad_mask();
      |                 ^~~~~~~~~~~~
In function ‘field_multiplier’,
    inlined from ‘u64_encode_bits’ at /home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:172:1,
    inlined from ‘u64p_replace_bits’ at /home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:172:1,
    inlined from ‘set_pmcr_n’ at arm64/vpmu_counter_access.c:50:2:
/home/itaru/projects/linux/tools/testing/selftests/../../../tools/include/linux/bitfield.h:136:17: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
  136 |                 __bad_mask();
      |                 ^~~~~~~~~~~~
arm64/vpmu_counter_access.c: At top level:
cc1: note: unrecognized command-line option ‘-Wno-gnu-variable-sized-type-not-at-end’ may have been intended to silence earlier diagnostics
make: *** [Makefile.kvm:303: /home/itaru/projects/linux/tools/testing/selftests/kvm/arm64/vpmu_counter_access.o] Error 1

Thanks,
Itaru.

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