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

[Michael S. Tsirkin]

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,
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.

-- 
MST