Re: Question about removing memslots

Avi Kivity
2012-03-28T10:05:40Z

On 03/28/2012 11:59 AM, Benjamin Herrenschmidt wrote:
> On Wed, 2012-03-28 at 11:37 +0200, Avi Kivity wrote:
>
> > > Now I see that x86 just seems to flush everything, which is quite heavy
> > > handed considering how often cirrus does it, but maybe it doesn't have a
> > > choice (lack of reverse mapping from GPA ?).
> >
> > We do have a reverse mapping, so we could easily flush just a single
> > slot. The reason it hasn't been done is that slot changes are very are
> > on x86. They're usually only done by 16-bit software; 32-bit software
> > just maps the entire framebuffer BAR and accesses it directly. It's
> > also usually done in a tight loop, so flushing everything doesn't have a
> > large impact (and with a 20-bit address space, you couldn't cause a
> > large impact if you wanted to - memory is all of 256 pages).
>
> Right ... except that it definitely seems to be happening here with
> cirrusfb in the guest kernel :-)
>
> Every time it does an imageblit it switches the BAR to emulation and
> back to direct mapping when the "upload" of the image is complete.

That's strange, the cirrus BAR allows the framebuffer and bitblt region
to coexist:

0000000000000000-7ffffffffffffffe (prio 0, RW): pci
00000000000a0000-00000000000bffff (prio 1, RW): cirrus-lowmem-container
00000000000a0000-00000000000a7fff (prio 1, RW): alias vga.bank0
@vga.vram 0000000000000000-0000000000007fff
00000000000a0000-00000000000bffff (prio 0, RW): cirrus-low-memory
00000000000a8000-00000000000affff (prio 1, RW): alias vga.bank1
@vga.vram

^ those are continuously flipped when running 16-bit software

0000000000008000-000000000000ffff
00000000000c0000-00000000000dffff (prio 1, RW): pc.rom
00000000000e0000-00000000000fffff (prio 1, R-): isa-bios
00000000fc000000-00000000fdffffff (prio 1, RW): cirrus-pci-bar0
00000000fc000000-00000000fc7fffff (prio 1, RW): vga.vram
00000000fc000000-00000000fc7fffff (prio 0, RW): cirrus-linear-io
00000000fd000000-00000000fd3fffff (prio 0, RW): cirrus-bitblt-mmio

^ the cirrus BAR, write to 0xfc000000 and you hit vga.vram, write to
0xfd000000 and you trigger a bitblt.

00000000feba0000-00000000febbffff (prio 1, RW): e1000-mmio
00000000febf0000-00000000febf0fff (prio 1, RW): cirrus-mmio

A guest driver problem perhaps?

> The X cirrus driver doesn't seem to trigger that (at least didn't from
> my limited testing so far) so it may not be using host data blits ...
> I'll have to compare what they do in more details.

Or maybe it understands the BAR layout better.

--
error compiling committee.c: too many arguments to function