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

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

Posted by Itaru Kitayama 2 weeks, 6 days ago

Signed-off-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
---
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));
 

---
base-commit: aae5a9834b388860844b294c70c8770dd26e528c
change-id: 20250912-selftest-fix3-27f285e79d82

Best regards,
-- 
Itaru Kitayama <itaru.kitayama@linux.dev>

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

Posted by Marc Zyngier 2 weeks, 6 days ago

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.

	M.

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

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

Posted by Itaru Kitayama 2 weeks, 6 days ago


> 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.

Thanks,
Itaru.

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

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

Posted by Marc Zyngier 2 weeks, 6 days ago

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.

	M.

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

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

Posted by Itaru Kitayama 2 weeks, 2 days ago


> 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.

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

Posted by Marc Zyngier 2 weeks ago

On Mon, 15 Sep 2025 22:31:31 +0100,
Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
> 
> 
> 
> > 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 bit field is *not* ignored when written from userspace. That's how
we configure the PMU if the guest runs at EL1.

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

[snip]

Can you please detail what compiler version this is? I'm unlikely to
install an ancient version of Ubuntu, but I can pick the corresponding
compiler version.

	M.

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

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

Posted by Itaru Kitayama 2 weeks ago


> On Sep 18, 2025, at 3:44, Marc Zyngier <maz@kernel.org> wrote:
> 
> On Mon, 15 Sep 2025 22:31:31 +0100,
> Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
>> 
>> 
>> 
>>> 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 bit field is *not* ignored when written from userspace. That's how
> we configure the PMU if the guest runs at EL1.
> 
>> The build error I’ve seen with Ubuntu 22.04 LTS is below:
> 
> [snip]
> 
> Can you please detail what compiler version this is? I'm unlikely to
> install an ancient version of Ubuntu, but I can pick the corresponding
> compiler version.
> 

Sure, here it is:

[itaru@vm4 ~]$ apt-cache show gcc
Package: gcc
Architecture: arm64
Version: 4:11.2.0-1ubuntu1
Priority: optional
Section: devel
Source: gcc-defaults (1.193ubuntu1)
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Debian GCC Maintainers <debian-gcc@lists.debian.org>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 50
Provides: c-compiler, gcc-aarch64-linux-gnu (= 4:11.2.0-1ubuntu1)
Depends: cpp (= 4:11.2.0-1ubuntu1), gcc-11 (>= 11.2.0-1~)
Recommends: libc6-dev | libc-dev
Suggests: gcc-multilib, make, manpages-dev, autoconf, automake, libtool, flex, bison, gdb, gcc-doc
Conflicts: gcc-doc (<< 1:2.95.3)
Filename: pool/main/g/gcc-defaults/gcc_11.2.0-1ubuntu1_arm64.deb
Size: 5128
MD5sum: 2ec2c9fcec3deb45052f307f85f37f7f
SHA1: 19491fe4fb89a6cbd389f573bf3abf46e841f3d6
SHA256: 426deed543cc32f388ad8336354e312fb76a47450194d2d775d855ad5878f82e
SHA512: c7e16c35037b0987458bf51a341045cdc1ac8da5bada4eb1c43922a83c5e18203beca8df9937b95de6c4185605f6e9054c92858d2842de0f6859cd21c0b33f01
Description-en: GNU C compiler
 This is the GNU C compiler, a fairly portable optimizing compiler for C.
 .
 This is a dependency package providing the default GNU C compiler.
Description-md5: c7efd71c7c651a9ac8b2adf36b137790
Task: ubuntustudio-video, ubuntustudio-publishing, ubuntu-mate-core, ubuntu-mate-desktop
Build-Essential: yes

Thanks,
Itaru.

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