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

[Gleb Natapov]

On Thu, Nov 18, 2010 at 04:04:14PM +0200, Michael S. Tsirkin wrote:
> > > What do you want to know?
> > How it claims access to framebuffer. Legacy VGA has not only IO space
> > but MMIO space too.
> 
> There's a separate bit to enable memory is that is what you
> are asking about.
> 
> The spec specifies the address ranges:
> 
>       Base class 03. Sub-class 00. Interface 000 0000b:
> 
>       VGA-compatible controller. Memory addresses 0A 0000h through 0B FFFFh.
>       I/O addresses 3B0h to 3BBh and 3C0h to 3DFh and all aliases of these
>       addresses.
> 
> 
So MMIO space is also not configurable? In short you can't insert two
vga card in two pci slots and use both simultaneously.

> > > > > > 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.
> > > > > 
> > > > That is possible.
> > > > 
> > > > > > 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.
> > > > > 
> > > > The code does it for both.
> > > 
> > > Yep. So it's a bug.
> > > 
> > > > > > > 
> > > > > > > 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 then only one card will be able to use IO since enabling IO on more
> > > > the one cards will cause conflict.
> > > 
> > > Sure. That's life for legacy io though.
> > > 
> > But that is the point. You can't have two regular VGA card
> > simultaneously.
> 
> You can't *enable* them simultaneously. The fact that we cant create
> them in qemu is a bug.
You can insert two of them on real HW too.

> 
> > The card should be designed to work in legacy
> > mode and non-legacy mode. Then one of them will be used by legacy
> > software like BIOS and another will be driven from an OS by a driver
> > written specifically for the card.
> 
> There's nothing in the spec that says so. IMO it
> should be possible to have two cards and have BIOS (or maybe even the
> OS) select which one to use.
Two use for what? If you have two cards you most probably want to use both
of them otherwise why have you put two of them in the same computer. But
only one of then can work in legacy mode and be usable without specialized
driver. This shouldn't be spelled out by the spec.

> 
> > > > > > 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.
> > > > Qemu can prevent any removal, surprise or not. Qemu can just
> > > > disallow device removal.
> > > 
> > > Yes, but that won't emulate real hardware faithfully.
> > To the letter. There is no HW with hot-unplaggable primary
> > vga card. You are welcome to surprise remove vga card from your
> > machine and see what will happen.
> 
> This is different from removing any other card with hotplug module
> unloaded in OS how?  OS might crash but so what? You can always reboot
> it, hardware won't be damaged, so qemu shouldn't crash too.
If guest can't handle unplug there is no meaning for qemu to provide it.
You can have development mobo were chipset can be unplugged. Should we
allow to hot-unplug chipset?

> 
> > > On real hardware with a hotplug supporting slot
> > > (and without an EM lock :) ) you can yank the card out
> > > and the guest can do nothing about it.
> > > 
> > And you will not find primary vga there.
> 
> Where? PCI spec explicitly allows VGA cards behind expansion slots,
> stick it there, and you can remove it too.
> 
You can't remove PCI card just from any PCI slot IIRC. Slot and add-on
card should be specifically designed to be hot-unpluggable. (Something
to do with when power and data lines are disconnected during un-plug
IIRC). Not all things you can yank from mobo while OS is running are
"unpluggable" :)

--
                        Gleb.