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Re: [Xen-devel] [PATCH] rombios: correct $PIR table router device ID

To: Keir Fraser <keir.xen@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] rombios: correct $PIR table router device ID
From: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Date: Tue, 31 May 2011 16:06:51 +0100
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On Tue, 2011-05-31 at 15:59 +0100, Keir Fraser wrote:
> On 31/05/2011 14:55, "Ian Campbell" <ian.campbell@xxxxxxxxxx> wrote:
> > Currently the $PIR table embedded in ROMBIOS refers to a "compatible PCI
> > Interrupt Router" with Vendor/Device ID 8086:122e (Intel 82371FB, Original
> > PIIX, I think), despite the fact that the Xen ioemu appears AFAICT to have
> > never provided this particular device.
> > 
> > Both the current qemu-xen and upstream qemu trees provide a device with ID
> > 8086:1237 (Intel 82441, i440FX chipset) so switch to using this.
> > 
> > Prior to 18931:0a393ba0ec0a the $PIR refered to an 8086:7000 (Intel 82371SB,
> > PIIX3), which again AFAICT nothing ever provided...
> > 
> > I've no idea which OSes actually pay attention to $PIR (Windows 98 perhaps?)
> > and I certainly don't know what behaviour either the old/wrong or the
> > new/"correct" value for this field will provoke. There's a strong arugment 
> > to
> > be made for leaving well enough alone...
> It doesn't have to be the *correct* device-id, just one that supports the
> same method for programming with PCI-ISA links. See
> http://www.microsoft.com/taiwan/whdc/archive/pciirq.mspx.

Ah so an OSes won't (or isn't supposed to) go looking for an actual
device with that specific ID but rather uses it as a key into its table
of ways of dealing with PCI-ISA links to find an alternative driver. I
should have read the document closer (in fairness the reality of these
semantics seem pretty unimaginable!).

> If we change this we risk referring to a too-modern chipset for which old
> OSes cannot find a matching driver. I suspect even the change to this in c/s
> 18931 (from upstream rombios) wasn't a tremendous idea.
> Bear this in mind for seabios too. ;-)

That was really why I posted this patch ;-) I'll stick with the old ID


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