Re: [Xen-devel] [PATCH 3/3] qemu-xen-trad: IGD passthrough: Expose vendor specific pci cap on host bridge.

[Ross Philipson <ross.philipson@xxxxxxxxxx>]

On 07/15/2013 12:06 PM, Pasi KÃrkkÃinen wrote:
> On Mon, Jul 01, 2013 at 09:06:37AM -0400, Konrad Rzeszutek Wilk wrote:
> > > > > > > Or just scan through the capabilities, and chain only the ones
> > > > > > > that we want to "Whitelist" and the rest are to be blacklisted.
> > > > > > > The rest can also have its values set to some bogus value (0xdeadbeef?)
> > > > > > > Perhaps only when built with 'debug=y'.
> > > > > >
> > > > > > That sounds about right. Back when I first did the patch (in an old qemu)
> > > > > > there were no other capabilities on the piix4 host bridge so it was simple.
> > > > > > Not sure if other capabilities will be an issue now.
> > > > > It's still the case as for the IVB host bridge.
> > > > > And from what I can find from the datasheet for the Haswell, it's
> > > > > still the case.
> > > > >
> > > > > Note that the datasheet explicitly documents the offset of the
> > > > > CAPABILITY registers.
> > > > > I guess there will be code that rely on this offset that been publicly
> > > > > documented.
> > > > >
> > > > > Btw. Ross, now that you appears to be the original author (sorry for
> > > > > mess you up with Jean),
> > > > > could you also comment on my rework proposal? Jan believe the current
> > > > > form is not clean enough.
> > > > >
> > > > > Currently we use a whitelist of registers to pass-through.How do you
> > > > > come up with the current list?
> > > > > The shadow copy way appears to work for the current list.
> > > > OK.
> > > > > But what if we are going to need some special registers that cannot be
> > > > > handled well? (e.g. has side effect for reading and cannot perform
> > > > > read-back?)
> > > > Hopefully the i915 driver in Linux will help in figuring out which
> > > > ones of those are needed?
> > > I remember the vendor cap fix only helps windows guest.
> > How was that diagnosed? Perhaps that information can be part of the source
> > code to help in the future with diagnosiing which caps are needed and
> > which ones can be blacklisted?
> I guess that's a question mostly for Ross/Jean as they're the original authors 
> of the patch?
We discovered the issue with Windows guests running the vendor drivers 
for the passed in IGD graphics device. Under certain circumstances 
(resuming from S3/S4 IIRC), the guest would BSOD. I finally tracked it 
down to a bad state in the resuming driver because it was not coded to 
handle the vendor capabilities not being present on the host bridge. 
BTW, those capabilities are flags indicating what features the IGD card 
has - their exact meaning is of course proprietary.
I cannot say it was only a problem on Windows but rather that that is 
the only place we ever saw it.
I never saw any other capabilities on the hosts bridges at that time, 
just vendor ones so the patch just handled that. If there were other 
capabilities, I would think it would have to be determined on a case by 
case basis whether they were included. Inclusion of each new type would 
have different ramifications it seems.
> -- Pasi

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