From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47991) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V7hF6-0006W5-BO for qemu-devel@nongnu.org; Fri, 09 Aug 2013 03:36:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V7hEy-0007EN-Dh for qemu-devel@nongnu.org; Fri, 09 Aug 2013 03:36:12 -0400 Received: from ozlabs.org ([203.10.76.45]:57559) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V7hEy-0007E1-2T for qemu-devel@nongnu.org; Fri, 09 Aug 2013 03:36:04 -0400 From: Rusty Russell In-Reply-To: <5203C62C.2040303@suse.de> References: <1375938949-22622-1-git-send-email-rusty@rustcorp.com.au> <1375938949-22622-2-git-send-email-rusty@rustcorp.com.au> <87li4cgvh1.fsf@codemonkey.ws> <5203AB19.9070505@suse.de> <87r4e4p4wj.fsf@codemonkey.ws> <5203C62C.2040303@suse.de> Date: Fri, 09 Aug 2013 17:05:49 +0930 Message-ID: <87eha3nwoa.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/7] virtio: allow byte swapping for vring and config access List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Andreas =?utf-8?Q?F=C3=A4rber?= , Anthony Liguori Cc: Peter Maydell , qemu-devel@nongnu.org Andreas F=C3=A4rber writes: > Am 08.08.2013 17:40, schrieb Anthony Liguori: >> Andreas F=C3=A4rber writes: >>> Am 08.08.2013 15:31, schrieb Anthony Liguori: >>>> We have a mechanism to do weak functions via stubs/. I think it would >>>> be better to do cpu_get_byteswap() as a stub function and then overload >>>> it in the ppc64 code. >>> >>> If this as your name indicates is a per-CPU function then it should go >>> into CPUClass. Interesting question is, what is virtio supposed to do if >>> we have two ppc CPUs, one is Big Endian, the other is Little Endian. >>=20 >> PPC64 is big endian. AFAIK, there is no such thing as a little endian >> PPC64 processor. >>=20 >> This is just a processor mode where loads/stores are byte lane swapped. >> Hence the name 'cpu_get_byteswap'. It's just asking whether the >> load/stores are being swapped or not. > > Exactly, just read it as "is in ... Endian mode". On the CPUs I am more > familiar with (e.g., 970), this used to be controlled via an MSR bit, > which as CPUPPCState::msr exists per CPUState. I did not check on real > hardware, but from the QEMU code this would allow for the mixed-endian > scenario described above. > >> At least for PPC64, it's not possible to enable/disable byte lane >> swapping for individual CPUs. It's done through a system-wide hcall. > > What is offending me is only the following: If we name it > cpu_get_byteswap() as proposed by you, then its first argument should be > a CPUState *cpu. Its value would be read from the derived type's state, > such as the MSR bit in the code path that you wanted duplicated. The > function implementing that register-reading would be a hook in CPUClass, > with a default implementation in qom/cpu.c rather than a fallback in > stubs/. To access CPUClass, CPUState cannot be NULL, as brought up by > Stefano for cpu_do_unassigned_access(); not following that pattern > prevents mixing CPU architectures, which my large refactorings have > partially been about. Cf. my guest-memory-dump refactoring. > > If it is just some random global value, then please don't call it > cpu_*(). Since sPAPR is not a target of its own, I don't see how/where > you want to implement that hcall query as per-target function either, > that might rather call for a QEMUMachine hook? > > I don't care or argue about byte lanes here, I am just trying to keep > API design consistent and not regressing on the way to heterogeneous > emulation. That's a lot of replumbing and indirect function calls for a fairly obscure case. We certainly don't have a nice CPUState lying around in virtio at the moment, for example. I can try to plumb this in if there's consensus, but I suspect it's making the job 10x harder. (The next logical step would be for st* and ld* to take the cpu to query its endianness, Anthony's weird ideas notwithstanding). Cheers, Rusty.