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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.04.15 at 13:19, <mst@xxxxxxxxxx> wrote:
> Yes Linux can't fix firmware 1st mode, but
> PCI express spec says what firmware should do in this case:
> 
> IMPLEMENTATION NOTE Software UR Reporting Compatibility with 1.0a Devices
> 
>         With 1.0a device Functions, 96 if the Unsupported Request Reporting 
> Enable bit is set, the Function
>         when operating as a Completer will send an uncorrectable error 
> Message (if enabled) when a UR
>         error is detected. On platforms where an uncorrectable error Message 
> is handled as a System Error,
>         this will break PC-compatible Configuration Space probing, so 
> software/firmware on such
>         platforms may need to avoid setting the Unsupported Request 
> Reporting Enable bit.
>         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 
> detected with posted Requests,
>         helping avoid this case for potential silent data corruption.
>         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.
> 
> 
> What I think you have is a very old 1.0a system, and you set Unsupported
> Request Reporting Enable.
> 
> Can you confirm?

No. In at least one of the two cases we got reports of the original
problem, triggering the finding of this issue, this is a brand new one,
only soon to become available publicly. Furthermore I'm being
confused by the mention of PC-compatible config space probing
above: The URs we talk about here don't result from config space
accessed at all.

> You will have other problems if your firmware doesn't follow the spec. So 
> how about either
> 
> - Don't use firmware 1st mode with pci express
>   (Seems no reason to do firmware 1st for PCIE, architecture is completely
>    standard. I saw mentions of using combined/parallel mode, using AER for 
> some
>    devices but not others, but I don't know how this is supposed to be 
> enabled.
>    Any idea?)
> 
> or
> 
> - ask your vendor to update firmware if it doesn't do the right thing

Both not very practical suggestions, based on experience.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel