Re: [Xen-devel] [PATCH] replace bogus -ENOSYS uses

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.08.16 at 12:34, <george.dunlap@xxxxxxxxxx> wrote:
> On 11/08/16 19:10, Andrew Cooper wrote:
>> On 09/08/16 11:40, Jan Beulich wrote:
>>> --- a/xen/arch/x86/cpu/mtrr/main.c
>>> +++ b/xen/arch/x86/cpu/mtrr/main.c
>>> @@ -332,7 +332,7 @@ int mtrr_add_page(unsigned long base, un
>>>     if ((type == MTRR_TYPE_WRCOMB) && !have_wrcomb()) {
>>>             printk(KERN_WARNING
>>>                    "mtrr: your processor doesn't support 
>>> write-combining\n");
>>> -           return -ENOSYS;
>>> +           return -EOPNOTSUPP;
>> 
>> Will this break the classic-xen MTRR code?  ISTR it was very picky, from
>> the cpuid work.  Also, as some further cleanup, that printk should
>> become a print-once.
>> 
>> The others look ok.
> 
> That does bring up a good general point though -- the return value is
> part of the ABI.  Are you reasonably confident that none of the callers
> will be confused when this return value changes?  If so, a note in the
> commit message justifying this confidence would be helpful I think.

I don't think specific return values can be considered part of the
ABI, or else we couldn't e.g. change the order in which certain
checks get performed.

And then please also consider a hypothetical future hypervisor with
the MTRR operations simply ripped out - that would return -ENOSYS
or -EOPNOTSUPP then too, without a way for the caller to tell that
more generic error condition from the more specific one here.

Jan


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