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

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Message-ID: <50D1AD38.7070606@redhat.com>
Date:	Wed, 19 Dec 2012 13:04:08 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Stefan Hajnoczi <stefanha@...hat.com>
CC:	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, mst@...hat.com,
	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

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

> Stefan
> 

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