Re: [Qemu-devel] [Qemu-ppc] qemu-system-ppc video artifacts since "tcg: drop global lock during TCG code execution"

[Gerd Hoffmann]

  Hi,

> Instead of having MMIO register spaces we use the dirty tracking
> mechanism. Here regions are marked as for dirty tracking and when the
> SoftMMU helper first comes to this bit of memory it will follow the slow
> path and mark region as visited.

visited?  Do you mean dirty bit is set for the page?  Or is this
something else?

> Once done this bit is cleared and all
> future writes to that page are written directly from the translated
> code. This no longer has an implicit synchronisation from the BQL so
> there is now a race and you can have memory being updated which might
> miss this flagging.

Not fully following what you are trying to say.  But pages being updated
without dirty bit getting set (if clear) certainly is a problem for the
vga emulation (and live migration too).

> Note that KVM has some similar hacks to avoid trapping all writes to
> video memory with its coalesced mmio mechanism however I'm not familiar
> with all the details.

Normal linear framebuffer access doesn't use this.

> Now there are mechanisms we can use to ensure there are no races happen
> and return to the situation that the display is only updated when the
> TCG cores are not running.

tcg and display updates running in parallel isn't a problem, we have
that with kvm anyway.  Dirty bit handling must be correct though.

With kvm at the start of each display update vga fetches the dirty
bitmap from the kernel (memory_region_sync_dirty_bitmap).  Then it goes
use memory_region_get_dirty to figure which pages have been touched.

When memory_region_sync_dirty_bitmap is called the kernel will clear the
memory bitmap of the region and also map all pages read-only.  Next
guest update will pagefault and the kernel can set the dirty bit for the
page (maybe there is a more efficient way with EPT available).

I suspect the memory_region_sync_dirty_bitmap call on tcg should reset
the fast path optimization, so the slow path can update the dirty bits
correctly.

cheers,
  Gerd