Re: [PATCH 2/2] x86/pci: Remove unnecessary check in VF value computation

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 02/18/2014 12:04 PM, Jan Beulich wrote:
>>>> On 18.02.14 at 17:56, Boris Ostrovsky <boris.ostrovsky@oracle.com> wrote:
>> On 02/18/2014 11:41 AM, Jan Beulich wrote:
>>>>>> On 18.02.14 at 16:49, Boris Ostrovsky <boris.ostrovsky@oracle.com> wrote:
>>>> On 02/18/2014 05:16 AM, Jan Beulich wrote:
>>>>>>>> On 13.02.14 at 10:48, "Jan Beulich" <JBeulich@suse.com> wrote:
>>>>>>>>> On 12.02.14 at 22:05, Boris Ostrovsky <boris.ostrovsky@oracle.com> wrote:
>>>>>>> This test is already performed a couple of lines above.
>>>>>> Except that it's the wrong code you remove:
>>>>> No opinion on this alternative at all?
>>>> Sorry Jan, I didn't realize you were waiting for me on this.
>>>>
>>>> Yes, your version is fine although to be honest I don't see how the
>>>> original patch had any issues with division by zero since we'd still be
>>>> inside the 'if (stride)' clause.
>>> It's the very division that this patch removes:
>>>
>>>>>> --- a/xen/arch/x86/msi.c
>>>>>> +++ b/xen/arch/x86/msi.c
>>>>>> @@ -635,7 +635,7 @@ static u64 read_pci_mem_bar(u16 seg, u8
>>>>>>                 return 0;
>>>>>>             base = pos + PCI_SRIOV_BAR;
>>>>>>             vf -= PCI_BDF(bus, slot, func) + offset;
>>>>>> -        if ( vf < 0 || (vf && vf % stride) )
>>>>>> +        if ( vf < 0 )
>>>>>>                 return 0;
>>>>>>             if ( stride )
>>>>>>             {
>>> Which isn't inside the if(stride).
>>
>> Yes, I see it now. I was staring at a wrong line.
>>
>> This actually now looks like a bug.
> You mean the old code looks wrong or the new one?


The old one. Your patch fixes it.


>
>> You do check above for '(num_vf > 1
>> && !stride) ' but presumably if things are really messed up num_vf can
>> be 1 but vf is 0. And then if stride is zero too then we are not doing
>> particularly well.
>>
>> So probably this should go into 4.4 as well?
> We've done with this unfixed quite fine so far, so it's generally
> okay to leave as is until 4.4.1 (read: not a regression). I
> personally wouldn't mind pushing it in, but only if other similar
> not too high priority bug fixes would also go in.
>

OK.

-boris