linux-kernel - Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

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Date:	Wed, 19 Dec 2012 18:51:30 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	gaowanlong@...fujitsu.com, hutao@...fujitsu.com,
	linux-scsi@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	asias@...hat.com, nab@...ux-iscsi.org
Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of
 buffers

On Wed, Dec 19, 2012 at 01:04:08PM +0100, Paolo Bonzini wrote:
> Il 19/12/2012 11:47, Stefan Hajnoczi ha scritto:
> > On Tue, Dec 18, 2012 at 01:32:48PM +0100, Paolo Bonzini wrote:
> >> +/**
> >> + * virtqueue_start_buf - start building buffer for the other end
> >> + * @vq: the struct virtqueue we're talking about.
> >> + * @buf: a struct keeping the state of the buffer
> >> + * @data: the token identifying the buffer.
> >> + * @count: the number of buffers that will be added
> > 
> > Perhaps count should be named count_bufs or num_bufs.
> 
> Ok.
> 
> >> + * @count_sg: the number of sg lists that will be added
> > 
> > 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).
> 
> 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.
> 
> Paolo

Hmm it should work with vhost.

> 
> > Stefan
> > 
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