Am 08.08.2013 17:40, schrieb Anthony Liguori: > Andreas Färber <afaer...@suse.de> 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. > > PPC64 is big endian. AFAIK, there is no such thing as a little endian > PPC64 processor. > > 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. Regards, Andreas > FWIW, I think most bi-endian architectures are this way too so I think > this is equally applicable to ARM. Peter, is that right? > > Regards, > > Anthony Liguori > >> We'd need to check current_cpu then, which for Xen is always NULL. >> >> Andreas -- SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg
