Re: [PATCH] libxc/PM: correct (not just) error handling in xc_get_cpufreq_para()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2025 11:51, Anthony PERARD wrote:
> On Thu, Mar 27, 2025 at 02:32:04PM +0100, Jan Beulich wrote:
>> HWP work made the cleanup of the "available governors" array
>> conditional, neglecting the fact that the condition used may not be the
> 
> I don't know why the cleanup was made conditional, as long as the bounce
> buffer is declared with DECLARE_NAMED_HYPERCALL_BOUNCE(),
> xc_hypercall_bounce_post() can be called without issue (even if
> ..bounce_pre isn't used). Maybe it's all those "unlock_*" labels that is
> misleading, a single "out" label which does the cleanup
> unconditionally would be more than enough.

Oh, yet more cleanup to do there (independently of course).

>> condition that was used to allocate the buffer (as the structure field
>> is updated upon getting back EAGAIN). Throughout the function, use the
>> local variable being introduced to address that.
>>
>> --- a/tools/libs/ctrl/xc_pm.c
>> +++ b/tools/libs/ctrl/xc_pm.c
>> @@ -212,31 +212,32 @@ int xc_get_cpufreq_para(xc_interface *xc
>>      DECLARE_NAMED_HYPERCALL_BOUNCE(scaling_available_governors,
>>                       user_para->scaling_available_governors,
>>                       user_para->gov_num * CPUFREQ_NAME_LEN * sizeof(char), 
>> XC_HYPERCALL_BUFFER_BOUNCE_BOTH);
>> -
>> -    bool has_num = user_para->cpu_num &&
>> -                     user_para->freq_num &&
>> -                     user_para->gov_num;
>> +    unsigned int in_gov_num = user_para->gov_num;
>> +    bool has_num = user_para->cpu_num && user_para->freq_num;
>>  
>>      if ( has_num )
> 
> I think there's an issue here, this condition used to be true if
> `gov_num` was not 0, even if `cpu_num` and `freq_num` was 0. That's not
> the case anymore. Shouldn't `has_num` use also the value from
> `gov_num`? Do we actually need `has_num`?

Raising this question is where all of this started:
https://lists.xen.org/archives/html/xen-devel/2025-03/msg01870.html.
IOW with Penny's change I think the need for has_num will disappear, the
latest. At this point, requesting the governors being optional, only
->gov_num shouldn't affect has_num. Once requesting frequencies becomes
optional too, has_num would become a mere alias of ->cpu_num.

Jan