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Re: [Xen-devel] intel IGD driver intel_detect_pch() failure

On Tue, Dec 11, 2012 at 8:01 PM, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Tue, 11 Dec 2012, G.R. wrote:
> >
> > On Tue, Dec 11, 2012 at 2:15 AM, Kay, Allen M <allen.m.kay@xxxxxxxxx> wrote:
> >       My understanding is that i915 driver needs to looks at the real PCH's device ID to apply HW workarounds.
> >
> >       One way to fix this is to make the device ID of the first PCH's (00:01.0) device ID reflect the device ID of
> >       00:1f.0 on the host.  This way, when i915 running as a guest will find the valid PCH device ID to make workaround
> >       decisions with.
> >
> >
> >       I don't know why it would make a difference if i915 is built into the kernel or as a module though.
> >
> >       Allen
> >
> > Thanks Allen for your input.
> > But module v.s. built-in is not the only difference. Another difference is the PVHVM build vs. pure HVM build.
> > Both share the same PCI layout but different result. Any theory how to explain the difference? What makes the PVHVM version
> > work?
> Please don't use HTML in emails.

I'm using gmail and not sure how to control html formatting.
I explicitly use 'remove formatting' this time, does it look better now?
Sorry for the inconvenience.
> PVHVM Linux guests setup interrupts differently: they request an event
> channel for each legacy interrupt or MSI/MSI-X, then the hypervisor uses
> these event channels to inject notifications into the guest, rather
> than emulated interrupts or emulated MSIs.
Will this affect the result of pci_get_class() as called by the intel driver?
If not, this can still not explain the different behavior.
Maybe I need to do one more experiment when I got time.
> Reading again the description of the bug, wouldn't it be better to just
> the fix the problem in Linux?
> In fact this looks like a bug in intel_detect_pch(): QEMU is emulating a
> PCI-PCI bridge and the driver is checking for an PCI-ISA bridge (to help
> with virtualization?). Moreover it only checks the first PCI-ISA bridge.
> As far as I know Xen has never exposed a PCI-ISA bridge with vendor ==
> Intel to the guest.
But why do xen expose the PCI-ISA bridge in host as a PCI-PCI bridge, doesn't it sounds strange?
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