On Wed, Aug 26, 2015 at 9:47 PM, Richard Henderson <r...@twiddle.net> wrote: > On 08/26/2015 09:17 AM, Artyom Tarasenko wrote: >> >> After some debugging I think it's caused by memory faults. On every >> MMU miss / access fault >> TB is re-translated multiple times till the faulting instruction is found. > > > That shouldn't happen. Are you certain it's not multiple MMU misses/faults?
You are right. These are multiple faults. >> AFAICT we produce data/access faults only on load/store instructions, i.e. >> if GET_FIELD(insn, 0, 1) == 3. Can this knowledge be used to reduce >> the number of re-translations? > > > No. > > From the fault, we have a host address where the fault occured. We then > retranslate the TB looking for what guest address corresponds to the code > generated at the host address. This is a one-pass process, not the multiple > passes you seem to be imagining. It also means we can't skip non-memory > insns during retranslation, as the host addresses would no longer line up. Right, thanks for clarifying this. I was confused by the multiple "Search PC..." messages. But I see now that one message corresponds to one translated instruction, not to one translation block. The log message should probably be moved one level up, and btw it's not sparc-specific, this part seems to be a copy-pasta in multiple targets. > That said, sun4u is a software managed tlb, which requires *lots* more extra > faults than a hardware managed tlb. In the later case, we can perform the > page table lookup and then continue the memory instruction without faulting. > > I think that implementing sun4v, with (most of) the hypervisor actually > within qemu, is the only way to get good performance for Sparc. I thought about it. The guest can profit from the knowledge it is executed virtualized in multiple ways. The problem with this approach is that Linux/sparc64 is currently not the primary target OS for me. And the legacy OSes do not support sun4v. Even a Solaris 8 has a quite limited support for it. Artyom -- Regards, Artyom Tarasenko SPARC and PPC PReP under qemu blog: http://tyom.blogspot.com/search/label/qemu
