[PATCH 00/10] rm/bochs: Modernize driver

[Gerd Hoffmann]

  Hi,

> Patch 8 replaces GEM VRAM with GEM SHMEM. The new memory manager
> is more reliable and allows for larger resolutions.

Valid point.

> Display updates were so slow that Gnome was unmanageable with a
> flickering cursor and single FPS. The new memory management makes
> Gnome at least useable.

Hmm?  I'm wondering where this huge improvement comes from?

With enough video memory VRAM performance should be ok.

If video memory is tight and ttm is forced to shuffle around
framebuffers between vram and system memory on each page flip (touching
much of vram along the way which causes additional overhead on the qemu
side), that is obviously very bad for performance.  One of the reasons
why cirrus uses SHMEM + shadowing since years.

Shadow buffering comes with some overhead too, so the switch isn't an
obvious win (assuming enough vram).  Hiding the page flips from qemu
might reduce the work qemu has to do though, especially if the shadowing
uses dirty tracking and only touches the vram pages which have actually
changed content.  So there is a fair chance that this outweighs the
shadowing overhead and ends up being a net win.  I don't expect the
difference being very big though.  Also different display usage patterns
might yield different results (fbcon vs. gnome for example).

So this probably makes sense, but I'd like to see a bit more background
information ...

On vram sizes:  The default qemu vram size (16M) should be fine for the
default display resolution (1280x800).  For FullHD vram size should be
doubled (-device VGA,vgamem_mb=32).


Skimmed the other patches, looks sane overall, but I don't follow drm
close enough any more to do an full review.  So I leave this here:

Acked-by: Gerd Hoffmann <kraxel at redhat.com>

take care,
  Gerd