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

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

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.

But as I said, either version is OK with me so you can add

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

-boris


>
> Jan
>
>>> @@ -639,11 +639,7 @@ static u64 read_pci_mem_bar(u16 seg, u8 bus, u8 slot, u8
>> func, u8 bir, int vf)
>>>           if ( vf < 0 || (vf && vf % stride) )
>>>               return 0;
>>>           if ( stride )
>>> -        {
>>> -            if ( vf % stride )
>>> -                return 0;
>>>               vf /= stride;
>>> -        }
>> Note how this second check carefully avoids a division by zero.
>>  From what I can tell I think that I simply forgot to remove the
>> right side of the earlier || after having converted it to the safer
>> variant inside the if(). Hence I think we instead want:
>>
>> x86/MSI: don't risk division by zero
>>
>> The check in question is redundant with the one in the immediately
>> following if(), where dividing by zero gets carefully avoided.
>>
>> Spotted-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>
>> --- 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 )
>>           {
>
>