Re: Ummunotify: progress at last!

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Tue, Mar 23, 2010 at 10:55:08PM -0700, Roland Dreier wrote:
>  > I would prefer to do this by adding a new verbs call that returns a fd
>  > directly. Ie use ib_uverbs_alloc_event_file and act like
>  > ibv_create_comp_channel.
>  > 
>  > The main reason for the new FD is so it can be polled on..
> 
> Agree, we don't want a new device node I don't think -- too hard to
> associate an fd you get from a separate open() with a uverbs context.

Right, that would suck..
 
>  > You can also avoid the mmap scheme by doing what perf events does,
>  > pass in a pointer from userspace and have the kernel pin that page it
>  > is on.
> 
> I wonder, is that a win?  I guess you don't even have to pin it, just do
> copy_to_user() to update the counter, but mmap doesn't seem so bad.

Not sure you can call copy_to_user from a mmu_notifier callback? What
if it faults?

I think the idea would be that by letting user space select the
counter location it could place it in a sensible cachline/etc and
probably avoid a pointer indirection.

Jason
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