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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/18/2014 11:41 AM, Jan Beulich wrote:
> > > > On 18.02.14 at 16:49, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 02/18/2014 05:16 AM, Jan Beulich wrote:
> > > > > > On 13.02.14 at 10:48, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> > > > > > > On 12.02.14 at 22:05, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 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 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?

-boris


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