[Qemu-devel] Re: sparc64 lazy conditional codes evaluation

[Blue Swirl <blauwirbel@gmail.com>]

On 5/3/10, Igor Kovalenko <igor.v.kovalenko@gmail.com> wrote:
> On Mon, May 3, 2010 at 11:24 PM, Blue Swirl <blauwirbel@gmail.com> wrote:
>  > On 5/3/10, Igor Kovalenko <igor.v.kovalenko@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);