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.
