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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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:

> @@ -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@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- 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 )
         {

Attachment: x86-MSI-VF-divide-by-zero.patch
Description: Text document

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