Re: [PATCH] i386/acpi: restore device paths for pre-5.1 vms

From: Igor Mammedov <imammedo@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-stable@nongnu.org, Vitaly Cheptsov <cheptsov@ispras.ru>,
	Thomas Lamprecht <t.lamprecht@proxmox.com>,
	qemu devel list <qemu-devel@nongnu.org>
Subject: Re: [PATCH] i386/acpi: restore device paths for pre-5.1 vms
Date: Fri, 26 Mar 2021 01:12:50 +0100	[thread overview]
Message-ID: <20210326011250.74fb7a1e@redhat.com> (raw)
In-Reply-To: <20210323152835-mutt-send-email-mst@kernel.org>

On Tue, 23 Mar 2021 15:32:23 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Mar 23, 2021 at 04:04:11PM +0100, Thomas Lamprecht wrote:
> > On 23.03.21 15:55, Vitaly Cheptsov wrote:  
> > >> 23 марта 2021 г., в 17:48, Michael S. Tsirkin <mst@redhat.com> написал(а):
> > >>
> > >> The issue is with people who installed a VM using 5.1 qemu,
> > >> migrated to 5.2, booted there and set a config on a device
> > >> e.g. IP on a NIC.
> > >> They now have a 5.1 machine type but changing uid back
> > >> like we do will break these VMs.
> > >>
> > >> Unlikley to be common but let's at least create a way for these people
> > >> to used these VMs.
> > >>  
> > > They can simply set the 5.2 VM version in such a case. I do not want to   
> > let this legacy hack to be enabled in any modern QEMU VM version, as it violates ACPI specification and makes the life more difficult for various other software like bootloaders and operating systems.
> > 
> > Yeah here I agree with Vitaly, if they already used 5.2 and made some configurations
> > for those "new" devices they can just keep using 5.2?  
> 
> People just create a VM, the machine type gets set in stone then.
> 
> > If some of the devices got configured on 5.1 and some on 5.2 there's nothing we can
> > do anyway, from a QEMU POV - there the user always need to choose one machine version
> > and fix up the device configured while on the other machine.  
> 
> I think I did not explain it well. Here is an example.
> 
> 
> 1. start qemu version 5.1 with machine type 5.1
> 
> 2. reboot with qemu version 5.2 still machine type 5.1
> 
> 3. set static IP on NIC
> 
> 4. reboot with qemu version 6.0 still machine type 5.1
> 
>    at this point NIC loses the static IP.
> 
> 
> 
> What I suggest is that we use a property to at stage 4,

Question is how users would know about fixup property, nobody knows about it
beside new QEMU.
Or even better how to make it transparent to user,
so the only action user would have to do is to upgrade to the newest QEMU.

> user can set a flag and avoid losing the static IP.
> Yes in theory maybe at stage 2 or 4 user can switch to 5.2 machine type
> but we were always telling people not to do that, let's not start.
> 