From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52359) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgvU7-0008Jg-Vi for qemu-devel@nongnu.org; Thu, 23 Feb 2017 10:39:12 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cgvU7-00062x-4B for qemu-devel@nongnu.org; Thu, 23 Feb 2017 10:39:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36626) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cgvU6-00062W-U8 for qemu-devel@nongnu.org; Thu, 23 Feb 2017 10:39:11 -0500 Date: Thu, 23 Feb 2017 08:39:08 -0700 From: Alex Williamson Message-ID: <20170223083908.657bf257@t450s.home> In-Reply-To: <2565ef99-9c16-e836-08c6-0915f5d4b0f8@redhat.com> References: <1487659615-15820-1-git-send-email-xyjxie@linux.vnet.ibm.com> <5edff645-12e8-d3e0-1849-302b6986c232@ozlabs.ru> <5a0773de-6bc7-474a-82ab-2edd37ce8a93@redhat.com> <92580ca9-47fe-a943-7720-d3cb1fc6d2eb@redhat.com> <3d5e7b5e-4501-86b7-093d-47fb09af585e@redhat.com> <41630a89-e645-7d7e-b7c2-356fd6dcadee@redhat.com> <2565ef99-9c16-e836-08c6-0915f5d4b0f8@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Peter Maydell , Alexey Kardashevskiy , Yongji Xie , QEMU Developers , zhong@linux.vnet.ibm.com, David Gibson , Paul Mackerras On Thu, 23 Feb 2017 16:21:47 +0100 Paolo Bonzini wrote: > On 23/02/2017 15:35, Peter Maydell wrote: > > On 23 February 2017 at 12:53, Paolo Bonzini wrote: > >> > >> > >> On 23/02/2017 13:26, Peter Maydell wrote: > >>> On 23 February 2017 at 11:43, Paolo Bonzini wrote: > >>>> On 23/02/2017 12:34, Peter Maydell wrote: > >>>>> We should probably update the doc comment to note that the > >>>>> pointer is to host-endianness memory (and that this is not > >>>>> like normal RAM which is target-endian)... > >>>> > >>>> I wouldn't call it host-endianness memory, and I disagree that normal > >>>> RAM is target-endian---in both cases it's just a bunch of bytes. > >>>> > >>>> However, the access done by the MemoryRegionOps callbacks needs to match > >>>> the endianness declared by the MemoryRegionOps themselves. > >>> > >>> Well, if the guest stores a bunch of integers to the memory, which > >>> way round do you see them when you look at the bunch of bytes? > >> > >> You see them in whatever endianness the guest used. > > > > I'm confused. I said "normal RAM and this ramdevice memory are > > different", and you seem to be saying they're the same. I don't > > think they are (in particular I think with a BE guest on an > > LE host they'll look different). > > No, they look entirely the same. The only difference is that they go > through MemoryRegionOps instead of memcpy. Is this true for vfio use case? If we use memcpy we're talking directly to the device with no endian conversions. If we use read/write then there is an endian conversion in the host kernel.