Re: [Qemu-devel] allocation zone extensions for the firmware linker/loader

[Michael S. Tsirkin]

On Tue, Jun 06, 2017 at 08:10:17PM +0200, Laszlo Ersek wrote:
> On 06/05/17 18:02, Michael S. Tsirkin wrote:
> > On Sat, Jun 03, 2017 at 09:36:23AM +0200, Laszlo Ersek wrote:
> >> On 06/02/17 17:45, Laszlo Ersek wrote:
> >>
> >>> The patches can cause linker/loader breakage when old firmware is booted
> >>> on new QEMU. However, that's no problem (it's nothing new), the next
> >>> release of QEMU should bundle the new firmware binaries as always.
> >>
> >> Dave made a good point (which I should have realized myself, really!),
> >> namely if you launch old fw on old qemu, then migrate the guest to a new
> >> qemu and then reboot the guest on the target host, within the migrated
> >> VM, things will break.
> >>
> >> So that makes this approach dead in the water.
> >>
> >> Possible mitigations I could think of:
> >> - Make it machine type dependent. Complicated (we don't usually bind
> >> ACPI generation to machine types) and wouldn't help existing devices.
> >> - Let the firmware negotiate these extensions. Very complicated (new
> >> fw-cfg files are needed for negotiation) and wouldn't help existing 
> >> devices.
> > 
> > This last option *shouldn't* be complicated. If it is something's wrong.
> > 
> > Maybe we made a mistake when we added etc/smi/*features*.
> > 
> > It's not too late to move these to etc/*features* for new
> > machine types if we want to and if you can do the firmware
> > work. Then you'd just take out a bit and be done with it.
> > 
> > I don't insist on doing the ACPI thing now but I do think
> > infrastructure for negotiating extensions should be there.
> 
> Different drivers in the firmware would need to negotiate different
> questions / features with QEMU independently of each other. The "thing"
> in OVMF that negotiates (and uses) the SMI broadcast is very-very
> different and separate from the "thing" in OVMF that handles the ACPI
> linker/loader script.


They both could call a common library.

Also, we don't need separate fw cfg files - we could
reserve ranges of bits in a single file.
E.g. bits 0-31 - smi, 32-63 - tseg, etc.

-- 
MST