Re: [PATCH] ioeventfd: Introduce KVM_IOEVENTFD_FLAG_PIPE

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 04, 2011 at 01:27:45PM +0300, Avi Kivity wrote:
> I'd like to be able to forward an entire PCI BAR
> over a pipe.  That means sending the address/data/length tuple, and
> both read and write support.
> 
> >>  Is the write guaranteed atomic?  We probably need serialization here.
> >
> >afaik vfs_write is just a wrapper to the write() function of the
> >underlying fs so it should be atomic, no?
> 
> write() isn't atomic in general.  It is for pipes under certain
> circumstances, but there is no guarantee that the circumstances
> apply, or that the fd is in fact a pipe.

So the above makes it tricky to pass structured data
which is > 1 byte in size over a pipe. We could build an in-kernel API
(not sure how useful it would be for userspace) to do
an atomic write or fail (instead of a partial write),
but we'll still need to handle failures.

-- 
MST