Xserver needs to run as "root" on Linux / was: Re: [Xorg] Server side widgets

John Dennis jdennis at redhat.com
Mon Jul 12 12:41:39 PDT 2004

On Mon, 2004-07-12 at 12:44, Jon Smirl wrote:
> As far as I
> know there is no absolute requirement to be root if the appropriate
> pieces get pushed in the device driver.

I'm still curious how in this model the performance issues that brought
the concept of "direct rendering" into existence will be addressed.

However, I do believe the issue is significantly mitigated with current
generation devices in which the GPU executes large DMA buffers of
rendering commands where the dispatch of these buffers occurs in a
driver with root privileges. By the way, this more typifies a 3D client
than what occurs with a 2D windowing system client in which large
batching is counter to expectations.

Maintaining optimal performance while balancing the balance of labor
between a driver and user level rendering process is tricky. How do you
propose to migrate things to a driver without degrading performance in
the general case?

The problem is that the transition between system and user is expensive
and if the operations performed by the driver are too small and fine
grained you're going to kill performance.
John Dennis <jdennis at redhat.com>

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