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

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Message-ID: <CAJSP0QUNQHSak_jUWA0jfDuKgfVFsyJb0252OxWzQzUcj653bg@mail.gmail.com>
Date:	Wed, 19 Dec 2012 13:40:57 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, "Michael S. Tsirkin" <mst@...hat.com>,
	hutao@...fujitsu.com, linux-kernel <linux-kernel@...r.kernel.org>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 1/5] virtio: add functions for piecewise addition of buffers

On Wed, Dec 19, 2012 at 1:04 PM, Paolo Bonzini <pbonzini@...hat.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
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