Re: [Qemu-devel] [PATCH] q35: fix mmconfig and PCI0._CRS

[Marcel Apfelbaum]

Hi Gerd,

On 5/29/19 8:01 AM, Gerd Hoffmann wrote:
> On Wed, May 29, 2019 at 07:48:03AM +0300, Marcel Apfelbaum wrote:
> > Hi Gerd,
> >
> > On 5/28/19 11:43 PM, Gerd Hoffmann wrote:
> > > This patch changes the handling of the mmconfig area.  Thanks to the
> > > pci(e) expander devices we already have the logic to exclude address
> > > ranges from PCI0._CRS.  We can simply add the mmconfig address range
> > > to the list get it excluded as well.
> > >
> > > With that in place we can go with a fixed pci hole which covers the
> > > whole area from the end of (low) ram to the ioapic.
> > >
> > > This will make the whole logic alot less fragile.  No matter where the
> > > firmware places the mmconfig xbar, things should work correctly.  The
> > > guest also gets a bit more PCI address space (seabios boot):
> > >
> > >       # cat /proc/iomem
> > >       [ ... ]
> > >       7ffdd000-7fffffff : reserved
> > >       80000000-afffffff : PCI Bus 0000:00            <<-- this is new
> > >       b0000000-bfffffff : PCI MMCONFIG 0000 [bus 00-ff]
> > >         b0000000-bfffffff : reserved
> > >       c0000000-febfffff : PCI Bus 0000:00
> > >         f8000000-fbffffff : 0000:00:01.0
> > >       [ ... ]
> > >
> > > So this is a guest visible change.
> > Looks good to me, but shouldn't we use some compat
> > property to not affect previous machine versions?
> acpi table changes are typically not versioned, and IIRC the acpi tables
> are part of the live migration data stream so the tables will not change
> under the feet of the running guest even when it is migrated to another
> qemu version.

I wasn't "worried" only about migration, but also about the visible 
change in
the guests keeping the same machine type and upgrading QEMU.

I am not saying is a "big" issue since it will probably not affect the 
guests at all.
Upgrading QEMU will look like a firmware update or something.

Thanks,
Marcel

> cheers,
>    Gerd