Re: [Qemu-devel] [PATCH] q35/mch: implement extended TSEG sizes

[Michael S. Tsirkin]

On Fri, Jun 09, 2017 at 10:01:18PM +0200, Gerd Hoffmann wrote:
> On Fri, 2017-06-09 at 13:40 +0200, Paolo Bonzini wrote:
> > 
> > On 08/06/2017 21:55, Michael S. Tsirkin wrote:
> > > We don't have room anywhere in PCI config space. Laszlo makes
> > > argument
> > > why it's safe for this device based on spec but it's anyone's guess
> > > whether current and future software will follow spec.  In short,
> > > going
> > > anywhere near the emulated device has a potential to break some
> > > drivers.
> > 
> > There are no such drivers.  The MCH and PCH are only touched by the
> > firmware, not by the OS.
> 
> Yea.  That is *exactly* the reason why I think simply using the 0x50
> offset probably works fine in practice even though I suspect on
> physical hardware it might be some undocumented register.  Much of the
> stuff in the host bridge pci config space is firmware territory, and we
> run qemu specific firmware *anyway*.
> 
> cheers,
>   Gerd

To be specific, what I meant is a bit that tells guest that a
config space register is available, and lets host find out
that guest is going to use it.

This to ensure full forward and backward compatibility.

I agree a fw cfg file for a single bit seems like an overkill, that's
why I thought sharing feature files with SMI would be a good idea.

Do you see an issue with that?

-- 
MST