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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Laszlo Ersek <lersek@redhat.com>
Cc: SeaBIOS devel list <SeaBIOS@seabios.org>,
	qemu devel list <qemu-devel@nongnu.org>,
	edk2-devel-ml01 <edk2-devel@ml01.01.org>,
	Kevin O'Connor <kevin@koconnor.net>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Ben Warren <ben@skyportsystems.com>,
	Dongjiu Geng <gengdongjiu@huawei.com>,
	Igor Mammedov <imammedo@redhat.com>,
	"Jordan Justen (Intel address)" <jordan.l.justen@intel.com>,
	"Leif Lindholm (Linaro address)" <leif.lindholm@linaro.org>,
	Shannon Zhao <zhaoshenglong@huawei.com>,
	Stefan Berger <stefanb@linux.vnet.ibm.com>,
	Xiao Guangrong <guangrong.xiao@linux.intel.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Gerd Hoffmann <kraxel@redhat.com>
Subject: Re: [Qemu-devel] allocation zone extensions for the firmware linker/loader
Date: Mon, 5 Jun 2017 19:02:00 +0300	[thread overview]
Message-ID: <20170605185815-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <2ff62b03-d71b-93c4-79bf-10682b229758@redhat.com>

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.

-- 
MST