Re: [Qemu-devel] [RFC PATCH 1/3] target-i386: Add 486sx, old486, and old486sx CPU models

[H. Peter Anvin]

On 03/25/2013 12:05 PM, Eduardo Habkost wrote:
> On Mon, Mar 25, 2013 at 11:44:30AM -0700, H. Peter Anvin wrote:
>> On 03/25/2013 08:15 AM, Igor Mammedov wrote:
>>>>
>>>> Such changes have been rejected in the past (e.g., n270 Atom).
>>>> I personally wouldn't object to 486 changes, but I guess it should
>>>> rather be handled via Igor's CPU static properties that I have in my
>>>> review queue: The .model value would be set to 8 but the PC machine
>>>> would be changed alongside to set model = 0 for pc-1.4 and earlier.
>>> It doesn't relates to property refactoring nor to slim CPU sub-classes
>>> conversion either. So it could go in independently.
>>>
>>> But is this change safe from migration POV?
>>>
>>
>> Well, given that the CPU model presented is actually closer to a model 8
>> than a model 0 it probably is... but the real question is what would
>> cause someone to do migration of a 486 CPU model.
>>
>> The n270 issue is problematic, because right now "n270" can't actually
>> run software compiled for N270...
> 
> FWIW, I wouldn't mind too much if the maintainers decide to document 486
> and n270 as "migration-unsafe" and then knowingly break live-migration
> of those CPU models between qemu <= 1.3 and qemu >= 1.4. It's up to the
> maintainers to choose which way to go.
> 

The right thing, of course (and I believe that's where things are going)
is to unwind these descriptions at the time the VM is created; the
migration should implement the machine as it was launched.

If that isn't practical, then the right thing to do is probably to have
some kind of machine description conversion (so, say, "486" can be
converted to "486-1.3" containing the legacy description), but telling
people that -cpu n270 is something other than a real N270 that can't run
N270 software is user-hostile in the extreme.

It needs to be possible to fix bugs....

        -hpa