On Mon, May 3, 2010 at 11:54 PM, Blue Swirl <blauwir...@gmail.com> wrote: > On 5/3/10, Igor Kovalenko <igor.v.kovale...@gmail.com> wrote: >> On Mon, May 3, 2010 at 11:24 PM, Blue Swirl <blauwir...@gmail.com> wrote: >> > On 5/3/10, Igor Kovalenko <igor.v.kovale...@gmail.com> wrote: >> >> Hi! >> >> >> >> There is an issue with lazy conditional codes evaluation where >> >> we return from trap handler with mismatching conditionals. >> >> >> >> I seldom reproduce it here when dragging qemu window while >> >> machine is working through silo initialization. I use gentoo minimal cd >> >> install-sparc64-minimal-20100322.iso but I think anything with silo boot >> >> would experience the same. Once in a while it would report crc error, >> >> unable to open cd partition or it would fail to decompress image. >> > >> > I think I've also seen this. >> > >> >> Pattern that fails appears to require a sequence of compare insn >> >> possibly followed by a few instructions which do not touch conditionals, >> >> then conditional branch insn. If it happens that we trap while >> processing >> >> conditional branch insn so it is restarted after return from trap then >> >> seldom conditional codes are calculated incorrectly. >> >> >> >> I cannot point to exact cause but it appears that after trap return >> >> we may have CC_OP and CC_SRC* mismatch somewhere, >> >> since adding more cond evaluation flushes over the code helps. >> >> >> >> We already tried doing flush more frequently and it is still not >> >> complete, so the question is how to finally do this once and right :) >> >> >> >> Obviously I do not get the design of lazy evaluation right, but >> >> the following list appears to be good start. Plan is to prepare >> >> a change to qemu and find a way to test it. >> >> >> >> 1. Since SPARC* is a RISC CPU it seems to be not profitable to >> >> use DisasContext->cc_op to predict if flags should be not evaluated >> >> due to overriding insn. Instead we can drop cc_op from disassembler >> >> context and simplify code to only use cc_op from env. >> > >> > Not currently, but in the future we may use that to do even lazier >> > flags computation. For example the sequence 'cmp x, y; bne target' >> > could be much more optimal by changing the branch to do the >> > comparison. Here's an old unfinished patch to do some of this. >> > >> >> Another point is that we always write to env->cc_op when >> >> translating *cc insns >> >> This should solve any issue with dc->cc_op prediction going >> >> out of sync with env->cc_op and cpu_cc_src* >> > >> > I think this is what is happening now. >> > >> >> 2. We must flush lazy evaluation back to CC_OP_FLAGS in a few cases when >> >> a. conditional code is required by insn (like addc, cond branch etc.) >> >> - here we can optimize by evaluating specific bits (carry?) >> >> - not sure if it works in case we have two cond consuming insns, >> >> where first needs carry another needs the rest of flags >> > >> > Here's another patch to optimize C flag handling. It doesn't pass my >> > tests though. >> > >> >> b. CCR is read by rdccr (helper_rdccr) >> >> - have to compute all flags >> >> c. trap occurs and we prepare trap level context (saving pstate) >> >> - have to compute all flags >> >> d. control goes out of tcg runtime (so gdbstub reads correct value >> from env) >> >> - have to compute all flags >> > >> > Fully agree. >> >> >> Cool >> >> Still I'd propose to kill dc->cc_op, find a reliable way to test it >> and then add it back possibly with more optimizations. >> I'm lost in the code up to the point where I believe we need to >> save/restore cc_op and cpu_cc* while switching trap levels. > > I'd think this should do the trick: > > diff --git a/target-sparc/op_helper.c b/target-sparc/op_helper.c > index b27778b..94921cd 100644 > --- a/target-sparc/op_helper.c > +++ b/target-sparc/op_helper.c > @@ -3506,6 +3506,8 @@ void do_interrupt(CPUState *env) > } > tsptr = cpu_tsptr(env); > > + helper_compute_psr(); > + > tsptr->tstate = ((uint64_t)GET_CCR(env) << 32) | > ((env->asi & 0xff) << 24) | ((env->pstate & 0xf3f) << 8) | > GET_CWP64(env); >
Thanks, this change seems to work here for silo issue. Another change would be to flush for gdbstub use of GET_CCR and for helper_rdccr. I tried to embed flush into GET_CCR but the code looks ugly since we need to proxy a call to helper_compute_psr from gdbstub passing available env pointer. Not really tested with your changes, but still what is the breakage you see? -- Kind regards, Igor V. Kovalenko
