Re: [Xen-devel] [v5][PATCH 4/5] xen, gfx passthrough: create host bridge to passthrough

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 1 Jul 2014, Chen, Tiejun wrote:
> On 2014/7/1 3:42, Stefano Stabellini wrote:
> > On Mon, 30 Jun 2014, Chen, Tiejun wrote:
> > > On 2014/6/29 20:14, Michael S. Tsirkin wrote:
> > > > On Sun, Jun 29, 2014 at 03:56:10PM +0800, Chen, Tiejun wrote:
> > > > > On 2014/6/27 19:26, Paolo Bonzini wrote:
> > > > > > Il 27/06/2014 10:34, Chen, Tiejun ha scritto:
> > > > > > > 
> > > > > > > 
> > > > > > > So how to separate this to specific to xen? Or you mean we need to
> > > > > > > create an new machine to address this scenario? But actually this
> > > > > > > is
> > > > > > > same as xenfv_machine except for these little codes.
> > > > > > 
> > > > > > Yes, please create a new machine so that "-M pc" doesn't have any of
> > > > > > these hacks.
> > > > > 
> > > > > But regardless of the machine is 'xenfv' or 'pc', we always call
> > > > > pc_init_pci(), then inside, i440fx_init() is always performed. So I
> > > > > think
> > > > > even we create a new machine, shouldn't we still call pc_init_pci()?
> > > > > 
> > > > > > 
> > > > > > Note that "-M xenfv" is obsolete, Xen can now use "-M pc" (i.e. the
> > > > > > default).
> > > > > > 
> > > > > 
> > > > > Yes, Xen can use 'pc'.
> > > > > 
> > > > > Thanks
> > > > > Tiejun
> > > > 
> > > > You are creating a new machine type where the pci host
> > > > looks like MCH but a bunch of other things are from i440fx.
> > > 
> > > Anthony,
> > > 
> > > Any comments to address this in xen case?
> > 
> > Having a separate machine for this is reasonable. You can select the
> > right one from libxl anyway. From the name point of view I would go with
> > "xenigd".
> 
> Do this mean we need to change something libxl?

Yes: you would need to pass a different -machine option, see
tools/libxl/libxl_dm.c:libxl__build_device_model_args_new.
It would be a pretty small change.

> > Just take a look at xenfv and introduce something similar but specific
> > to your IGD PT use case.
> > 
> 
> Which PCIe machine should I refer? i440fx or q35?

I'd say i440fx.


> Thanks
> Tiejun
> 
> > 
> > > Thanks
> > > Tiejun
> > > 
> > > > 
> > > > I have some doubts about this combination being worth supporting - it
> > > > does not seem useful for anything except xen from the code you posted,
> > > > but maybe you can whittle down the number of places where you poke at
> > > > the host to make it reasonable: I can imagine that, if you are lucky and
> > > > the registers that i915 wants to poke to make it work on real hardware
> > > > happen to fall on top of reserved registers in the i440FX/PIIX3 pci
> > > > bridge.  OTOH it would be much more likely if you just start with
> > > > something that does have MCH, like Q35, or emulate a newer
> > > > machine type.  This is the path that people who wanted
> > > > to boot iOS on QEMU took, and the result is pretty good.
> > > > 
> > > > But regardless, this is clearly not a i440fx nor a q35 pc
> > > > so it needs a separate name.
> > > > 
> > > 
> > 
> 

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