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Date: Fri, 9 Oct 2009 08:48:09 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCHv2] seabios: enable io/memory unconditionally
Message-ID: <20091009064809.GB9942@redhat.com>
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To: Kevin O'Connor <kevin@koconnor.net>
Cc: Anthony Liguori <aliguori@us.ibm.com>, avi@redhat.com, kvm-devel <kvm@vger.kernel.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On Thu, Oct 08, 2009 at 10:29:53PM -0400, Kevin O'Connor wrote:
> Hi Michael,
> 
> On Thu, Oct 08, 2009 at 05:53:46PM +0200, Michael S. Tsirkin wrote:
> > VGA adapters need to claim memory and i/o
> > transactions even if they do not have any
> > i/o or memory bars. E.g. PCI spec, page 297,
> > gives an example of such a device:
> > 
> >     Programming interface 0000 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.
> > 
> > While we could check for these devices and special-case them, it is
> > easier to fix this by enabling i/o and memory space unconditionally:
> > devices that do not support it will just ignore this setting.
> 
> This doesn't sound correct to me - I would think the vga option rom
> should enable the memory and io bars.

You can do this, but in real systems, BIOS enables memory on all
devices. Just take a real system and do lspci -v on a device that has no
driver loaded.
And note how seabios does this *already* - it just doesn't
enable I/O for VGA device unless it has I/O bar, which is wrong.

>  I don't have enough knowledge
> to say for sure though - can someone else with knowledge in this area
> confirm this approach?
> 
> -Kevin