From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37787) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgqMC-0005BI-UX for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:10:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cgqM7-0003Sw-V5 for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:10:40 -0500 Received: from mx1.redhat.com ([209.132.183.28]:32886) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cgqM7-0003Sd-LA for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:10:35 -0500 References: <1487659615-15820-1-git-send-email-xyjxie@linux.vnet.ibm.com> <5edff645-12e8-d3e0-1849-302b6986c232@ozlabs.ru> From: Paolo Bonzini Message-ID: <5a0773de-6bc7-474a-82ab-2edd37ce8a93@redhat.com> Date: Thu, 23 Feb 2017 11:10:31 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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: Peter Maydell Cc: Alexey Kardashevskiy , Yongji Xie , QEMU Developers , Alex Williamson , zhong@linux.vnet.ibm.com, David Gibson , Paul Mackerras On 23/02/2017 11:02, Peter Maydell wrote: > On 23 February 2017 at 08:35, Paolo Bonzini wrote= : >> >> >> On 23/02/2017 05:20, Alexey Kardashevskiy wrote: >>> First, Paolo is right and ram_device_mem_ops::endianness should be >>> host-endian which happens to be little in our test case (ppc64le) >> >> So you tested a ppc64 BE guest and it works? >> >>> Keep things where they are in the VFIO department and just fix >>> ram_device_mem_ops::endianness? >> >> I would fix the ram_device_mem_ops. Either by introducing >> DEVICE_HOST_ENDIAN(*) or with Yongji's patch. >> >> (*) DEVICE_NATIVE_ENDIAN is special cased all over the place >> because the same device (in a file that's compiled just once) >> can be either little- or big-endian. DEVICE_HOST_ENDIAN can >> be a simple #define to either DEVICE_LITTLE_ENDIAN or >> DEVICE_BIG_ENDIAN, because host endianness is the same for >> all QEMU binaries. It's literally half a dozen lines of code. >=20 > I'm really not convinced we need DEVICE_HOST_ENDIAN. RAM > areas should be target-endian (you can probably define > "target endianness" as "the endianness that RAM areas have".) This is not RAM. This is MMIO, backed by a MMIO area in the host. The MemoryRegionOps read from the MMIO area (so the data has host endianness) and do not do any further swap: data =3D *(uint16_t *)(mr->ram_block->host + addr); Here, the dereference is basically the same as ldl_he_p. If you wanted to make the MemoryRegion use DEVICE_NATIVE_ENDIAN, you'd need to tswap around the access. Or you can use ldl_le_p and DEVICE_LITTLE_ENDIAN (this is what Yongji's patch open codes), or ldl_be_p and DEVICE_BIG_ENDIAN. They are all the same in the end. Paolo