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

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

On Mon, Apr 13, 2015 at 01:40:59PM +0100, Jan Beulich wrote:
> >>> On 13.04.15 at 13:47, <mst@xxxxxxxxxx> wrote:
> > On Mon, Apr 13, 2015 at 12:34:34PM +0100, Jan Beulich wrote:
> >> >>> 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.
> > 
> > OK. Can you please explain why does UR cause a system error then?
> > It looks like a hardware bug: PCIE 1.1 seems to say it shouldn't.
> 
> Quite possible. Looking at the ITP log we were provided, the UR
> severity bit is clear (non-fatal), yet the error got surfaced to the
> OS as a fatal one (I would guess because it validly gets flagged as
> uncorrectable at the same time).
> 
> Jan

No, that's not valid.
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.

-- 
MST

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