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

[Avi Kivity]

On 09/14/2011 11:06 PM, Blue Swirl wrote:
> On Wed, Sep 14, 2011 at 8:35 AM, Avi Kivity<address@hidden>  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?

The current pci bridge implementation (440fx) uses an alias to 
instantiate pci 0xa0000-0xc0000 at the same address in the host address 
space.  If you disable it, those addresses map back to RAM - but there 
is no distinction between a BAR at that address and a VGA card at that 
address.

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