[Spice-devel] multiple heads memory arrangement

[Alon Levy]

On Wed, Apr 18, 2012 at 03:56:54PM -0600, Noel Van Hook wrote:
> I am not sure I fully understand the ramifications of having a list of ram
> headers.  Is the intent to have one additional ram header per additional
> head?

Yes, that was the idea.

> 
> For multiple heads to work correctly, I believe:
> 1) the framebuffer must be contiguous
> 2) the entire framebuffer must be accessible to all heads, and
> 3) each head must be able to be abitrarily located in that framebuffer.

Do you have any pointers to these? About #3 I understand, but I wasn't
aware of #1 or #2. But that said the framebuffer creation is already pretty
flexible with QXL. The guest issues a CreatePrimary IO and fills in the
header's (the specific head's one, actually this brings a good point -
it has to use different io ports - I guess I'll need to add a rom field
for that) create_primary field, which has a QXLPHYSICAL address of the
surface start, and a stride != width * bpp neccessarily. So it can
certainly have a single on pci framebuffer that overlaps all the heads
if it wants to - it's a guest issue, spice will never acccess it
directly but only via the primary surface for each head.

> 
> Unless I misunderstand what you are doing, I am concerned that multiple ram
> headers won't be flexible enough.

I think the above description takes care of that.

Thanks for the input,
Alon

> 
> Noel
> 
> 
> On Wed, Apr 18, 2012 at 4:53 AM, Alon Levy <alevy at redhat.com> wrote:
> 
> > Hi,
> >
> >  I'm just starting to work on this, wanted to solicit some feedback. I
> >  am thinking that the simplest way to implement multiple heads would be:
> >
> >  Add additional QXLRam headers back to back on the vram bar.
> >  Add a new flag QXL_RAM_FLAG_MORE_HEADS, set it on all but the last
> >  (largest offset into bar) header.
> >  Don't add new fields to QXLRom or QXLRam.
> >  Update QXLRom.ram_header_offset to point to the first (smallest
> >  offset) header.
> >
> > New driver that notices the QXL_RAM_FLAG_MORE_HEADS will count the
> > headers (it cannot be done by looking at the header offset since it may
> > not know about additional future data that will be put at the end of the
> > bar).
> >
> > Old driver will lose sizeof(QXLRam) * (num_heads - 1) bytes from the end
> > of the bar.
> >
> > Visually it would look like this for a two head qxl device:
> >
> >   .----------------------------------------------.
> >   |                   ROM BAR                    |
> >   | ...                                          |
> >  || ram_header_offset                            |
> >  || ...                                          |
> >  ||                                              |
> >  ||                                              |
> >  |'----------------------------------------------'
> >  |
> >  |
> >  |.--------------------------------------------------.
> >  ||                     VRAM BAR                     |
> >  |.--------------------------------------------------.
> >  ||                   Working area                   |
> >  ||                                                  |
> >  v.--------------------------------------------------.
> >   |                   First Header                   |
> >   | magic                                            |
> >   | ...                                              |
> >   | flags == QXL_RAM_FLAG_MORE_HEADS                 |
> >   |                                                  |
> >   .--------------------------------------------------.
> >   |                  Second Header                   |
> >   | magic                                            |
> >   | ... (identical fields) ...                       |
> >   | flags == 0                                       |
> >   '--------------------------------------------------'
> >
> > Alon
> > _______________________________________________
> > Spice-devel mailing list
> > Spice-devel at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/spice-devel
> >
> 
> 
> 
> -- 
> Noel Van Hook - Intelligraphics, Inc
> 522 Cooper Ave N
> Red Lodge, MT 59068
> Noel.Van.Hook at Intelligraphics.com  (425) 770-5561