Re: Graphics pass-through

From: Avi Kivity <avi@redhat.com>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: "Alex Williamson" <alex.williamson@redhat.com>,
	"Prasad Joshi" <prasadjoshi124@gmail.com>,
	"André Weidemann" <Andre.Weidemann@web.de>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Oswaldo Cadenas" <oswaldo.cadenas@gmail.com>,
	"Nikolaev, Maxim" <maxim.nikolaev@siemens.com>
Subject: Re: Graphics pass-through
Date: Wed, 11 May 2011 16:54:40 +0300	[thread overview]
Message-ID: <4DCA9520.2080207@redhat.com> (raw)
In-Reply-To: <4DCA9421.6060703@siemens.com>

On 05/11/2011 04:50 PM, Jan Kiszka wrote:
> On 2011-05-11 15:26, Avi Kivity wrote:
> >  On 05/11/2011 04:08 PM, Jan Kiszka wrote:
> >>  On 2011-05-11 13:25, Avi Kivity wrote:
> >>>   On 05/09/2011 06:48 PM, Alex Williamson wrote:
> >>>>>    That's an interesting trace! We'll check this here, but I bet it
> >>>>>    contributes to the problems. Our FX 3800 has 1G memory...
> >>>>
> >>>>   Yes, qemu leaves far too little MMIO space to think about assigning
> >>>>   graphics cards.  Both of my cards have 512MB and I hacked qemu to leave
> >>>>   a bigger gap via something like:
> >>>>
> >>>
> >>>   What about 64-bit BARs?
> >>
> >>  Aren't they backward compatible? Or do you think some guest drivers may
> >>  assume to find their 64-bit capable bars also registered as such and get
> >>  upset when seeing them as 32-bit ones?
> >>
> >
> >  I mean, if you have a 1GB framebuffer, put it above 4GB and hope the
> >  OS/driver can handle it.
>
> The question is if the drivers actually depend on this. At least the
> binary nvidia thing here on my notebook, it is obviously happy with
> below-4G-bars (and likely change the mapped window on demand):
>
> 01:00.0 VGA compatible controller: nVidia Corporation GT216 [Quadro FX 880M] (rev a2) (prog-if 00 [VGA controller])
>          Subsystem: Fujitsu Limited. Device 1584
>          Flags: bus master, fast devsel, latency 0, IRQ 16
>          Memory at cc000000 (32-bit, non-prefetchable) [size=16M]
>          Memory at d0000000 (64-bit, prefetchable) [size=256M]
>          Memory at ce000000 (64-bit, prefetchable) [size=32M]
>          I/O ports at 2000 [size=128]
>          [virtual] Expansion ROM at cd000000 [disabled] [size=512K]
>          Capabilities: [60] Power Management version 3
>          Capabilities: [68] MSI: Enable- Count=1/1 Maskable- 64bit+
>          Capabilities: [78] Express Endpoint, MSI 00
>          Capabilities: [b4] Vendor Specific Information: Len=14<?>
>          Capabilities: [100] Virtual Channel
>          Capabilities: [128] Power Budgeting<?>
>          Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024<?>
>          Kernel driver in use: nvidia
>
> Maybe the crashing Windows driver of the FX3800 has different
> requirements.

I doubt it.  A 64-bit BAR would be configured as 32-bit on an older 
BIOS, no?

I'd guess 64-bit BARs are only needed for large BARs.

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