Re: [Xen-devel] [PATCH][XSA-126] xen: limit guest control of PCI command register

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Mon, Apr 20, 2015 at 03:08:09PM +0100, Jan Beulich wrote:
> >>> On 20.04.15 at 15:43, <mst@xxxxxxxxxx> wrote:
> > On Mon, Apr 13, 2015 at 01:51:06PM +0100, Jan Beulich wrote:
> >> >>> On 13.04.15 at 14:47, <mst@xxxxxxxxxx> wrote:
> >> > Can you check device capabilities register, offset 0x4 within
> >> > pci express capability structure?
> >> > Bit 15 is 15 Role-Based Error Reporting.
> >> > Is it set?
> >> > 
> >> > The spec says:
> >> > 
> >> >  15
> >> >  On platforms where robust error handling and PC-compatible 
> >> > Configuration 
> >> > Space probing is
> >> >  required, it is suggested that software or firmware have the 
> >> > Unsupported 
> >> > Request Reporting Enable
> >> >  bit Set for Role-Based Error Reporting Functions, but clear for 1.0a 
> >> > Functions. Software or
> >> >  firmware can distinguish the two classes of Functions by examining the 
> >> > Role-Based Error Reporting
> >> >  bit in the Device Capabilities register.
> >> 
> >> Yes, that bit is set.
> > 
> > curiouser and curiouser.
> > 
> > So with functions that do support Role-Based Error Reporting, we have
> > this:
> > 
> > 
> >     With device Functions implementing Role-Based Error Reporting, setting 
> > the 
> > Unsupported Request
> >     Reporting Enable bit will not interfere with PC-compatible 
> > Configuration 
> > Space probing, assuming
> >     that the severity for UR is left at its default of non-fatal. However, 
> > setting the Unsupported Request
> >     Reporting Enable bit will enable the Function to report UR errors 97 
> > detected with posted Requests,
> >     helping avoid this case for potential silent data corruption.
> 
> I still don't see what the PC-compatible config space probing has to
> do with our issue.

I'm not sure but I think it's listed here because it causes a ton of URs
when device scan probes unimplemented functions.

> > did firmware reconfigure this device to report URs as fatal errors then?
> 
> No, the Unsupported Request Error Serverity flag is zero.
> 
> Jan

OK, that's the correct configuration, so how come the box crashes when
there's a UR then?

-- 
MST

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