Re: questions on having a driver pin user memory for DMA

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Thu, Apr 20, 2000 at 01:39:53AM -0500, Eric W. Biederman wrote:
> 
> The rules of thumb on this issue are:
> 1) Don't pin user memory let user space mmap driver memory.

map_user_kiobuf is intended to allow user space buffers to be mapped
the other way safely.

> 2) If you must have access to user memory use the evolving kiobuf
>    interface.  But that is mostly useful for the single shot
>    read/write case.  

There are not many problems with long-lived buffers mapped by kiobufs.
The fork problem is the main one, but we already have patches for that.

> I'm a little dense, with all of the headers and trailers
> that are put on packets how can it be efficient to DMA to/from
> user memory?  You have to look at everything to compute checksums
> etc.  

VIA != IP.

> Your interface sounds like it walks around all of the networking
> code in the kernel.  How can that be good?

VIA != networking.  VIA == messaging.  It provides for very (VERY) 
low latency user-space-to-user-space transfers, bypassing the O/S
entirely by allowing the O/S to grant the application direct, 
limited access to the HW control queues.

--Stephen

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