[vbox-dev] Synchronizing access to the framebuffer in VirtualBox 5

Rūdolfs Bundulis rudolfs.bundulis at gmail.com
Thu May 19 19:31:50 UTC 2016

Hi Vitali,

> the guest may access the guest VRAM between NotifyUpdate calls.
> Which means that in principle the guest can change the content while the
> frontend thread is reading pixels.

Yeah, it seems that this is what is happening. It can be easily observed
when looking at the mouse cursor in our video stream - you can see some
pixels of the previous ones. If there just was something equivalent to the
IFramebuffer::Lock() then we could just prevent the guest from accessing
the pixels (I mean it would still be for a very short time - only as long
as it takes for the pixel conversion). I mean my plan Z now is that I can
recompile VirtualBox and put back additional methods for locking - but I
was wondering don't you guys ever experience the same issues (however since
you simply blit the pixels to the display I guess not :D).

> Actually I wonder why this worked for your in VBox 4.3 because if the 4.3
> framebuffer implementation
> used the guest VRAM directly, then it was the same case as in 5.0.
In VBox 4.3 we performed the RGB to NV12 conversion inside the notify
update callback and we also locked an internal critical section which also
was locked if IFramebuffer::Lock was called - so during the time while our
frontend was reading the pixels noone else could access them. This was
safe, but slow as hell. I mean the responsiveness of the guest OS has
become much much much better now with the normalized fps encoding instead
of reacting to every update.

> Do you remember whether the 4.3 framebuffer implementation allocated the
> memory and did not use VRAM directly?
We always used VRAM. If the amount of memory wasn't so high we could use
our own pointer and copy, but in case of a full screen video or OpenGL
redirection where we won't be able to simply copy smaller regions this will
be a performance pitfall.

Best Regards,
Rudolfs Bundulis
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