On Wed, Dec 19, 2012 at 1:04 PM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 19/12/2012 11:47, Stefan Hajnoczi ha scritto: >> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
