Re: [PATCH 07/14] hw/i386/vmport: Add support for CMD_GETBIOSUUID

[Liran Alon]

On 10/03/2020 13:22, Michael S. Tsirkin wrote:
> On Tue, Mar 10, 2020 at 01:13:21PM +0200, Liran Alon wrote:
> > On 10/03/2020 11:34, Michael S. Tsirkin wrote:
> > > On Tue, Mar 10, 2020 at 01:54:04AM +0200, Liran Alon wrote:
> > > > This is VMware documented functionallity that some guests rely on.
> > > > Returns the BIOS UUID of the current virtual machine.
> > > >
> > > > Reviewed-by: Nikita Leshenko <address@hidden>
> > > > Signed-off-by: Liran Alon <address@hidden>
> > > So this at least seems guest-visible.
> > >
> > > So I suspect you need to add properties to
> > > disable this for old machine types, to avoid
> > > breaking compatibility with live-migration.
> > It is indeed guest visible.
> > In theory, you are right that for every guest-visible change, we should make
> > sure to expose it to only new machine-types.
> >
> > However, in this case, I feel it just unnecessary over-complicates the code.
> > I don't see how a guest which previously failed to use this command, will
> > fail because after Live-Migration it could succeed.
> The reverse can happen, start guest on a new qemu, command seems to
> work, then we migrate and it fails.
>
> And I guess this applies to the version right?
>
> > If you insist, I will add such functionality. In that case, do you think a
> > single flag will suffice for the addition of all new commands
> > (i.e. "commands-version" that it's number specifies set of commands to
> > expose), or you want to have a per-command flag?
> >
> > -Liran
> Can be a single flag but I'd just do it a boolean that enables a group
> of commands. E.g. "commands-v2".
Re-thinking about this...

QEMU VMPort interface was quite broken already (See first patch in 
series "hw/i386/vmport: Propagate IOPort read to vCPU EAX register").
The introduction of that fix already changes the result of all existing 
commands from guest perspective which relied on return-value from 
vmport_ioport_read().
E.g. CMD_GETVERSION and CMD_GETRAMSIZE.

In theory, we should have also made that bug-fix be tied to 
machine-type. To similarly avoid the issue of migrating a VM from a 
working VMPort command implementation to a non-working one.
i.e. In case of migrating from new QEMU to old QEMU. Do we wish to 
create a property-flag for that fix as-well? Or can we just drop all the 
machine-type flags alltogether (Including the suggested "commands-v2")
and declare this the first actually working VMPort implementation?

-Liran