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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> did firmware reconfigure this device to report URs as fatal errors then?

No, the Unsupported Request Error Serverity flag is zero.

Jan


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