From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53632) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0yOd-0006fJ-Nr for qemu-devel@nongnu.org; Tue, 16 Dec 2014 15:07:09 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Y0yOX-0000N2-JT for qemu-devel@nongnu.org; Tue, 16 Dec 2014 15:07:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51671) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y0yOX-0000Mx-B0 for qemu-devel@nongnu.org; Tue, 16 Dec 2014 15:06:57 -0500 Message-ID: <549090D8.5010006@redhat.com> Date: Tue, 16 Dec 2014 21:06:48 +0100 From: Laszlo Ersek MIME-Version: 1.0 References: <1418399932-7658-1-git-send-email-lersek@redhat.com> <1418399932-7658-2-git-send-email-lersek@redhat.com> <20141216134858.GD3283@hawk.usersys.redhat.com> <5490815E.8@redhat.com> <54908CB6.8030501@redhat.com> In-Reply-To: <54908CB6.8030501@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v4 1/8] fw_cfg: max access size and region size are the same for MMIO data reg List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini , Andrew Jones Cc: peter.maydell@linaro.org, Alexander Graf , qemu-devel@nongnu.org On 12/16/14 20:49, Paolo Bonzini wrote: > > > On 16/12/2014 20:00, Laszlo Ersek wrote: >> Yes. >> >> The root of this question is what each of >> >> enum device_endian { >> DEVICE_NATIVE_ENDIAN, >> DEVICE_BIG_ENDIAN, >> DEVICE_LITTLE_ENDIAN, >> }; > > Actually, I think the root of the answer :) is that fw_cfg_read (and > thus fw_cfg_data_mem_read) is not idempotent. The split/compose stuff > accesses the bytes at offsets 8,9,10,11,12,13,14,15 and composes them > according to the endianness. > > In the case of fw_cfg it just retrieves 8 bytes, but in the case of host > big endian it reads them in the "wrong" order for some reason (sorry, I > haven't looked at this thoroughly). I can't imagine how that would happen; fw_cfg_data_mem_read() ignores both "addr" and "size", and fw_cfg_read() simply advances the "cur_offset" member. > So the solution is: > > 1) make fw_cfg_data_mem_ops DEVICE_LITTLE_ENDIAN > > 2) make fw_cfg_data_mem_read and fw_cfg_data_mem_write call fw_cfg_read > and fw_cfg_write SIZE times and build up a value from the lowest byte up. Nonetheless, that's a really nice idea! I got so stuck with the automatic splitting that I forgot about the possibility to act upon the "size" parameter in fw_cfg_data_mem_read(). Thanks! ... Another thing that Andrew mentioned but I didn't cover in my other email -- what about fw_cfg_ctl_mem_ops? It's currently DEVICE_NATIVE_ENDIAN. You flipped the combined ops to LE in commit 6fdf98f2 (and, apparently, I reviewed it). Shouldn't we do the same for the standalone selector? Thanks Laszlo