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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Laszlo Ersek (lersek@redhat.com) 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.

Yep, sorry it always complicates things; as does the opposite case of
back-migrating a VM with an old machine type but newer bios to an old
qemu.

Dave

> 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.
> 
> So I guess I'll do what Igor and Gerd suggested: record in advance
> whether any pointer field narrower than 8 bytes points into a given
> blob, and if so, forbid allocating that blob from 64-bit address space.
> This should solve Ard's needs purely within the firmware.
> 
> Regarding the NOACPI hint, I guess I'm dropping that. I only meant
> NOACPI for addressing Igor's long-standing dislike for the "ACPI SDT
> header probe suppression" in VMGENID (and future similar devices). But,
> there's no actual *technical* need to eliminate that (unlike the
> technical need for 64-bit blob allocations which should really be
> solved), so I guess it's OK to postpone NOACPI indefinitely.
> 
> Self-nack for this set of sets.
> 
> Thanks for the feedback,
> Laszlo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK