Re: [Qemu-devel] [PATCH v5 7/7] pc: add PC_I440FX_COMPAT to disable aercap for vifo device

[Michael S. Tsirkin]

On Wed, Mar 18, 2015 at 08:15:01AM -0600, Alex Williamson wrote:
> On Wed, 2015-03-18 at 15:05 +0100, Michael S. Tsirkin wrote:
> > On Wed, Mar 18, 2015 at 08:02:26AM -0600, Alex Williamson wrote:
> > > On Wed, 2015-03-18 at 14:23 +0100, Michael S. Tsirkin wrote:
> > > > typo in subject: vfio, not vifo.
> > > > 
> > > > On Thu, Mar 12, 2015 at 06:23:59PM +0800, Chen Fan wrote:
> > > > > for piix4 chipset, we don't need to expose aer, so introduce
> > > > > PC_I440FX_COMPAT for all piix4 machines to disable aercap,
> > > > > and add HW_COMPAT_2_2 to disable aercap for all lower
> > > > > than 2.3.
> > > > > 
> > > > > Signed-off-by: Chen Fan <address@hidden>
> > > > 
> > > > Well vfio is never migrated ATM.
> > > > So why is compat code needed at all?
> > > 
> > > It's not for migration, it's to maintain current behavior on existing
> > > platforms.  If someone gets an uncorrected AER error on q35 machine type
> > > today, the VM stops.  With this change, AER would be exposed to the
> > > guest and the guest could handle it.  The compat change therefore
> > > maintains the stop VM behavior on existing q35 machine types.
> > 
> > If stop VM behaviour is useful, expose it to users.
> > If not, then don't.
> > I don't see why does it have to be tied to machine types.
> 
> Because q35-2.2 machine type will currently do a stop VM on uncorrected
> AER error.  If we don't tie that to a machine option then q35-2.2 would
> suddenly start exposing the error to the guest.  That's a fairly
> significant change in behavior for a static machine type.

I don't think you can classify it as a behaviour change. VM stop is not
guest visible behaviour.
Are you worrying about guests misbehaving when they see these errors?
Then you want this as user-controlled, supported option.

In other words: we only tie things to machine types when we
have to. This code gets almost no testing, and is a lot of
work to test. This one sounds like "just in case" is not a good
motivation.


-- 
MST