From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:41024)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <gkurz@linux.vnet.ibm.com>) id 1YDBmC-0007ei-0Y
	for qemu-devel@nongnu.org; Mon, 19 Jan 2015 07:49:53 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <gkurz@linux.vnet.ibm.com>) id 1YDBm7-0001nG-AL
	for qemu-devel@nongnu.org; Mon, 19 Jan 2015 07:49:51 -0500
Received: from e06smtp12.uk.ibm.com ([195.75.94.108]:50905)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <gkurz@linux.vnet.ibm.com>) id 1YDBm7-0001mr-0F
	for qemu-devel@nongnu.org; Mon, 19 Jan 2015 07:49:47 -0500
Received: from /spool/local
	by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use
	Only! Violators will be prosecuted
	for <qemu-devel@nongnu.org> from <gkurz@linux.vnet.ibm.com>;
	Mon, 19 Jan 2015 12:49:44 -0000
Received: from b06cxnps3074.portsmouth.uk.ibm.com
	(d06relay09.portsmouth.uk.ibm.com [9.149.109.194])
	by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id 9BA751B08070
	for <qemu-devel@nongnu.org>; Mon, 19 Jan 2015 12:49:42 +0000 (GMT)
Received: from d06av08.portsmouth.uk.ibm.com (d06av08.portsmouth.uk.ibm.com
	[9.149.37.249])
	by b06cxnps3074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with
	ESMTP id t0JCnfEY56623322
	for <qemu-devel@nongnu.org>; Mon, 19 Jan 2015 12:49:41 GMT
Received: from d06av08.portsmouth.uk.ibm.com (localhost [127.0.0.1])
	by d06av08.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with
	ESMTP id t0JCnfro031429
	for <qemu-devel@nongnu.org>; Mon, 19 Jan 2015 05:49:41 -0700
Date: Mon, 19 Jan 2015 13:49:37 +0100
From: Greg Kurz <gkurz@linux.vnet.ibm.com>
Message-ID: <20150119134937.6c58c242@bahia.local>
In-Reply-To: <CAFEAcA-r0S5-rtovfyG9h+TPUFbnnawVSY74K-rso49TwFs_7g@mail.gmail.com>
References: <CAJSP0QU9YbTaXGbJRcx8qCdqF0yBdWj276KkhGVN1WV37bRTNQ@mail.gmail.com>
	<CAFEAcA-r0S5-rtovfyG9h+TPUFbnnawVSY74K-rso49TwFs_7g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] virtio_tswaps() vs virtio_ld/st_p()
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>

On Mon, 19 Jan 2015 11:45:39 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On 19 January 2015 at 11:35, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > What is the difference between the virtio_tswaps() and
> > virtio_ld/st_p() families of functions?
> 
> virtio_ld/st*_p() perform a load or store from memory pointed
> to by the (host) pointer argument.
> 
> virtio_tswap16 &c take a value, swap it as appropriate and
> return a value.
> 
> virtio_tswap16s &c do an in-place byteswap on the value pointed
> to by the host pointer argument.
> 
> (The tswap*/tswap*s pairs are a mirror of the equivalent families
> of functions bswap16/bswap16s provided by bswap.h.)
> 
> The most important distinction here is that the ld/st functions
> will handle possibly unaligned accesses, whereas the tswap
> functions will not. The former are intended for "fish this value
> out from some guest RAM" and the latter for "we have a structure
> we've copied from guest RAM already via memcpy and want to swizzle
> the entries in it".
> 
> -- PMM
> 

Heh, not sure I would have provided such a comprehensive answer. :)

Stefan,

Feeling curious, what was the intent behind your question ?

Cheers.

--
Greg