Re: [Qemu-devel] Re: [PATCH] spice: add qxl device

[Michael S. Tsirkin]

On Thu, Nov 18, 2010 at 11:57:51AM +0200, Gleb Natapov wrote:
> On Thu, Nov 18, 2010 at 11:30:27AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 18, 2010 at 11:11:49AM +0200, Gleb Natapov wrote:
> > > On Thu, Nov 18, 2010 at 11:03:21AM +0200, Michael S. Tsirkin wrote:
> > > > On Thu, Nov 18, 2010 at 10:09:35AM +0200, Gleb Natapov wrote:
> > > > > On Wed, Nov 17, 2010 at 08:00:08PM +0200, Michael S. Tsirkin wrote:
> > > > > > > >>  If so: does qemu
> > > > > > > >>emulate this correctly?
> > > > > > > >
> > > > > > > >It mostly does.
> > > > > > > 
> > > > > > > I doubt it actually enables/disables the legacy vga ports.
> > > > > > 
> > > > > > I'll check when I have the time. We can fix it if it doesn't,
> > > > > > 
> > > > > So many guests (all of them?) just assume that vga ports and
> > > > > framebuffer is there.
> > > > 
> > > > Why do you think they disable io memory then?
> > > > 
> > > Who and how and when disables io memory?
> > 
> > I think guest will do this if you disable the device through the device
> > manager. This might need a reboot to become effective.
> > 
> Try to do it with primary VGA adapter and tell us what happens :)
> 
> > > Some guests are designed to run
> > > even on old ISA machines that have no way to disable anything. The
> > > device is just there.
> > > 
> > > This is the same with IDE ports. BIOS "knows" legacy ISA ports and just
> > > program them into PCI IO bars to be nice.
> > 
> > HAven't checked IDE, for VGA AFAIK BIOS does not program legacy ports in
> > the card, they are hardwired there. However, the card must not claim any
> > io transactions if IO memory is disabled in command register.
> > 
> Is this correct also for legacy ports?

Yes. The spec is quite explicit on this point:

A function that supports a PC legacy function (IDE, VGA, etc.) is
allowed to claim those addresses associated with the specific function
when the I/O Space (see Figure 6-2) enable bit is set.  These addresses
are not requested using a Base Address register but are assigned by
initialization software. If a device identifies itself as a legacy
function (class code), the initialization software grants the device
permission to claim the I/O legacy addresses by setting the device’s I/O
Space enable bit.


> This wouldn't be backwards
> compatible to ISA machines, so old software my not run properly back in
> the days when transaction from ISA to PCI happened.

initialization software could be the BIOS.
So maybe BIOS update was needed in the transition.

> So my guess is that
> old ISA ports works in backwards compatible way.

The spec seems to contradict this.

> > When qemu is started, it works correctly: the io memory is disabled and 
> > card does
> > not claim any io. Then BIOS comes along and enables io. At this point
> > map callback is invoked and maps io memory, card starts claiming io.
> Looking at the code I see that cirrus claims all IO ports and
> framebuffer memory during init function unconditionally.

So that may be OK for ISA, but not for PCI.

> > 
> > What is broken is that if BIOS/guest then disables IO memory,
> > (I think - even if guest is rebooted!) we will keep claiming IO 
> > transactions.
> > That our emulation does this seems to be a clear spec violation, we are
> > just lucky that BIOS/guest does not do this at the moment.
> > 
> > 
> > 
> > > > > So what "fixing" this will buy us?
> > > > 
> > > > Besides spec compliancy, you mean?  Ability to support multiple VGA
> > > > cards. That's how it works I think: BIOS enables IO on the primary
> > > > VGA device only.
> > > > 
> > > What spec defines hot-plug for primary VGA adapter?
> > 
> > No idea about hotplug. I am talking about multiple VGA cards,
> > enabling/disabling them dynamically should be possible.
> Of course. With properly designed VGA card you should be able to have
> more then one,

And, for that to have a chance to work when all cards are identical, you
don't claim IO when IO is disabled.

> but one of them will provide legacy functionality
> and is not removable.

The guest might not support hotplug. But there's no way
it can prevent surprise removal. qemu should not crash
when this happens.

> > 
> > > Our BIOS should support
> > > -M isa machine too.
> > 
> > > There is no way to disable VGA or even check if it
> > > is present there.
> > 
> > So I guess with isa you can't have multiple VGA cards work.  With PCI
> > I think you can.
> > 
> With isa you just need custom designed HW and software and you can have
> as many VGA cards as you want :)
> 
> --
>                       Gleb.