copyin()/copyout() constraints ?

[Luigi Rizzo]

On Sat, Jun 15, 2013 at 02:31:34PM +0300, Konstantin Belousov wrote:
> On Sat, Jun 15, 2013 at 01:26:10PM +0200, Luigi Rizzo wrote:
> > With "indirect buffers" in netmap/vale, i can eliminate the A->B copy,
> > and do A->C with a copyin in the kernel of the host.
> > But the per-packet budget is minuscule, and i am afraid that doing
> > an unconditional vslock() on each buffer is going to be too expensive
> > (and then i should also unwire the page ?
> 
> Using vslock() causes enormous user map fragmentation and should be
> avoided. The cost is basically the same as the page fault, since
> vslock() is a wrapper around vm_map_wire(), which just calls page fault
> handler for each address in region.
> 
> On the other hand, I do not understand how any kernel-side lock would
> prevent usermode from unmapping a buffer while other thread in kernel
> mode access it.

the lock i was talking about in previous emails protects
the in-kernel data structures (B, C and the VALE switch),
which are manipulated within system calls.

The user buffer A is not protected, i am happy to take
a page fault in the rare case it happens (and normally it won't
because in my application A has been written to just moments before).

cheers
luigi