Re: [Xen-devel] [Qemu-devel] [PATCH v3 11/11] igd: move igd-passthrough-isa-bridge creation to machine init

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 7 Jan 2016, Gerd Hoffmann wrote:
> On Do, 2016-01-07 at 13:10 +0000, Stefano Stabellini wrote:
> > CC'ing the Xen x86 maintainers
> > 
> > On Thu, 7 Jan 2016, Gerd Hoffmann wrote:
> > >   Hi,
> > > 
> > > > One thing I don't like about this is that it is going to skip the checks
> > > > done in xen_pt_initfn.
> > > 
> > > Hmm?  Those checks are still done when you assign a igd ...
> > 
> > Their failure doesn't affect the creation of the bridge.
> 
> Doesn't their failure makes qemu throw a fatal error and exit?
> So the guest isn't going to run either way?

No, it doesn't. QEMU doesn't even print an error message.


> > > > For example it is going to create the isa bridge,
> > > > even if there is going to be an error loading the vga bios or if the
> > > > device specified is not even an Intel graphic card.
> > > 
> > > Creating the special igd-isa-bridge is no longer tied to actually
> > > assigning a igd, but to the igd-passthru=on machine option being present
> > > (and machine type being 'pc').
> > 
> > and machine type 'xenfv', unless I am mistaken?
> 
> Yes, xenfv too (uses i440fx too and thus is a 'pc' derivate).

Good


> > > xen_pt_initfn checks that igd-passthru=on is set in case it finds a igd
> > > device is assigned, that will make sure the igd-isa-bridge is present.
> > > 
> > > But, yes, you can create a igd-isa-bridge now even when not assigning a
> > > igd device, either by specifying igd-passthru=on or using -device.  I
> > > fail to see why this is a problem though, care to explain?
> > 
> > It is going to change the PCI layout of any virtual machines with a
> > config file containing
> > 
> > gfx_passthru="igd"
> > 
> > and no pci config line. A Xen 4.7 user could add gfx_passthru="igd" to
> > all her VM config files, because actually it does nothing unless an
> > Intel graphic card is assigned to the VM.
> 
> No.  It changes the host bridge even when not passing through a igd,
> because that is linked to igd-passthru=on only.
>
> So making both host bridge tweak and isa bridge tweak triggered by
> igd-passthru=on brings more consistency to the whole thing.

That is true. Given that the only qemu-xen codebase with igd support is
4.7 and 4.7 hasn't been released yet, I am OK with changing the guest
visible PCI layout. I might ask for your help in backporting the patches
;-)


> > > Also note that moving this to machine init nicely handles the fact that
> > > the igd-isa-bridge is needed on 'pc' only, not on 'q35'.  If you don't
> > > want create the igd-isa-bridge in machine init, what is your alternative
> > > suggestion to handle this?
> > 
> > Maybe we could retain the check whether an Intel graphic card has been
> > assigned? 
> 
> Should be possible, but is not that easy due to initialization order
> issues.

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