Re: [Qemu-devel] [SeaBIOS] [PATCH] don't expose pvpanic device in the UI

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 06, 2013 at 12:32:47PM +0300, Gleb Natapov wrote:
> On Tue, Aug 06, 2013 at 12:21:48PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Aug 06, 2013 at 11:36:25AM +0300, Gleb Natapov wrote:
> > > On Tue, Aug 06, 2013 at 11:33:10AM +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Aug 06, 2013 at 10:21:52AM +0300, Gleb Natapov wrote:
> > > > > > If you see a mouse in a room, how likely is it that there's
> > > > > > a single mouse there?
> > > > > > 
> > > > > > This is a PV technology which to me looks like it was
> > > > > > rushed through and not only set on by default, but
> > > > > > without a way to disable it - apparently on the assumption
> > > > > > there's 0 chance it can cause any damage. Now that
> > > > > > we do know the chance it's not there, why not go back
> > > > > > to the standard interface, and why not give
> > > > > > users a chance to enable/disable it?
> > > > > You should be able to disable it with: -device pvpanic,ioport=0
> > > > 
> > > > Doesn't work for me.
> > > Bug that should be fixed. With this command line _STA should return
> > > zero.
> > 
> > It doesn't have anything to do with _STA: device still appears in QOM.
> You said disabled, not removed.

I really meant disable adding it.

> So does -global pvpanic,ioport=0
> disables the device for you?

What do you mean by disabled?

> > It's a QEMU issue, devices that are added with -device are
> > documented in -device help and removed by dropping them from
> > command line. Devices added by default have no way to
> > be dropped from QOM except -nodefaults.
> > 
> Are you saying that because pvpanic is added automatically QEMU -device
> help does not print help about it? Why not fix that? What QEMU --help
> issues has to do with deciding which devices should or should not be
> present by default?

No, I'm saying what I said: that there's no way to remove a device
added by default except -nodefaults, and no way to
find out what does -nodefaults exclude so you
can add things you need back selectively.

We wanted to fix the later issue for a long time, it's
hard to fix - that's why we don't fix that.
Just using -device for everything is a work-around for now.

> > > > Besides, both -device pvpanic and use of ioport=0 to disable it
> > > > are completely undocumented.
> > > > 
> > > Not the only undocumented thing in QEMU command line :)
> > 
> > All -device fields are documented with -device help.
> > This was supposed to be the right way to add
> > all new devices.
> > 
> > 
> > > > BTW pls keep qemu-devel Cc'd.
> > > > 
> > > Haven't touched CC list.
> > > 
> > > > > We have different definition of "damage" though.
> > > > 
> > > > Driver bugs, qemu bugs, OSPM bugs all can cause issues
> > > > like OS crashes, suspend/resume issues, bad
> > > > performance ... What's your definition of damage?
> > > > 
> > > None of those cover the case at hand.
> > 
> > Sigh. These examples demonstrate why would user want to run
> > QEMU without the pvpanic device.
> > 
> He can disable it, but chances the device will cause aforementioned
> problems are so much smaller (by design mind you) than PV APIC hotplug
> device that it makes me wonder why haven't you advocate to make PV APIC
> hotplug device to be configurable with -device too.

This might be a good idea for Q35.
We probably want to keep it as is for PIIX for compatibility.
Then again, different behaviour for Q35/PIIX might confuse people.

> --
> 			Gleb.