On Wed, Dec 19, 2012 at 1:04 PM, Paolo Bonzini [off-list ref] wrote:

Il 19/12/2012 11:47, Stefan Hajnoczi ha scritto:

quoted

On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote:
What is the purpose of count_sg?

It is needed to decide whether to use an indirect or a direct buffer.
The idea is to avoid a memory allocation if the driver is providing us
with separate sg elements (under the assumption that they will be few).

Ah, this makes sense now.  I saw it affects the decision whether to go
indirect or not but it wasn't obvious why.

Originally I wanted to use a mix of direct and indirect buffer (direct
if add_buf received a one-element scatterlist, otherwise indirect).  It
would have had the same effect, without having to specify count_sg in
advance.  The spec is not clear if that is allowed or not, but in the
end they do not work with either QEMU or vhost, so I chose this
alternative instead.

Okay.

Stefan