Re: blktap: Sync with XCP, dropping zero-copy.

[Daniel Stodden <daniel.stodden@citrix.com>]

On Wed, 2010-11-17 at 16:57 -0500, Daniel Stodden wrote:

> > > In any case, let's skip trying what happens if a thundering herd of
> > > several hundred userspace disks tries gfp()ing their grant slots out of
> > > dom0 without without arbitration.
> > 
> > I'm not against arbitration, but I don't think that's something that
> > should be implemented as part of a Xen driver.
> 
> Uhm, maybe I'm misunderstanding you, isn't the whole thing a Xen driver?
> What do you suggest?

Just misread you, sorry. You mean arbitration via IPC.

Somewhat, but ugly. Just counting pages with cmpxchg in some shm segment
isnt't a big deal, agreed. But userspace fixing the counter after
detecting some process crash would already start to complicate that.
Next, someone has to do the ballooning, and you need gntmap to
understand the VMA type anyway. From there on, going the rest of the way
and let the kernel driver round-robin the frame pool altogether is much
smaller and cleaner.

Daniel