Re: [Qemu-devel] [PATCH v3 5/6] vga: Use linear mapping + dirty logging in chain 4 memory access mode

[Avi Kivity <avi@redhat.com>]

On 09/14/2011 11:25 PM, Blue Swirl wrote:
> On Wed, Sep 14, 2011 at 8:15 PM, Avi Kivity<avi@redhat.com>  wrote:
> >  On 09/14/2011 11:06 PM, Blue Swirl wrote:
> >>
> >>  On Wed, Sep 14, 2011 at 8:35 AM, Avi Kivity<avi@redhat.com>    wrote:
> >>  >    On 09/14/2011 11:27 AM, Alexander Graf wrote:
> >>  >>
> >>  >>    On 14.09.2011, at 10:24, Jan Kiszka wrote:
> >>  >>
> >>  >>    >      On 2011-09-14 10:22, Avi Kivity wrote:
> >>  >>    >>      On 09/14/2011 11:20 AM, Jan Kiszka wrote:
> >>  >>    >>>>
> >>  >>    >>>>      Anyway PCI supports the vga region at 0xa0000-0xc0000.  Where
> >>  >>  is it
> >>  >>    >>>>      supposed to be mapped?
> >>  >>    >>>
> >>  >>    >>>      ...but not all PCI bridges make use of this feature / forward
> >>  >>  legacy
> >>  >>    >>>      requests.
> >>  >>    >>>
> >>  >>    >>
> >>  >>    >>      Then this should be fixed in the bridge?
> >>  >>    >
> >>  >>    >      Yes, it's a PPC bug.
> >>  >>
> >>  >>    So how does the bridge not forward it then?
> >>  >>
> >>  >
> >>  >    I expect that currently vga adds the region to pci_address_space().  We
> >>  >  need
> >>  >    to create a pci_address_space_vga() function that returns a region for
> >>  >  vga
> >>  >    to use.  Then add or remove the region to pci_address_space(), within
> >>  >  the
> >>  >    bridge code, depending on whether the bridge forwards vga accesses or
> >>  >  not.
> >>
> >>  Similar treatment should be also needed for VGA IO ports 0x3b0 etc.
> >>
> >>  >    (assuming I understood the problem correctly - not sure)
> >>
> >>  I think you did.
> >
> >  Maybe, but the solution can't be right.  The bridge can't distinguish
> >  between a BAR mapped at 0xa0000 and the vga device claiming accesses to
> >  0xa0000.  Is this what is happening?
>
> If VGA enable is set, the bridge will forward the accesses to VGA
> memory or ports to secondary interface. It doesn't care which device
> uses them. This is described in VGA support chapter in the PCI bridge
> spec.

That doesn't match the original description, where it appeared that the 
vga window collided with a BAR.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.