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[Qemu-devel] Re: sparc64 lazy conditional codes evaluation
From: |
Blue Swirl |
Subject: |
[Qemu-devel] Re: sparc64 lazy conditional codes evaluation |
Date: |
Tue, 4 May 2010 23:21:12 +0300 |
On 5/3/10, Igor Kovalenko <address@hidden> wrote:
> On Mon, May 3, 2010 at 11:54 PM, Blue Swirl <address@hidden> wrote:
> > On 5/3/10, Igor Kovalenko <address@hidden> wrote:
> >> On Mon, May 3, 2010 at 11:24 PM, Blue Swirl <address@hidden> wrote:
> >> > On 5/3/10, Igor Kovalenko <address@hidden> 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?
Aurora 2.0
(http://distro.ibiblio.org/pub/linux/distributions/aurora/build-2.0/sparc/iso/)
breaks.
This is what I get with git HEAD, having pressed enter key twice:
Welcome to Aurora SPARC Linux
+--------------+ CD Found +--------------+
| |
| To begin testing the CD media before |
| installation press OK. |
| |
| Choose Skip to skip the media test |
| and start the installation. |
| |
| +----+ +------+ |
| | OK | | Skip | |
| +----+ +------+ |
| |
| |
+----------------------------------------+
<Tab>/<Alt-Tab> between elements | <Space> selects | <F12> next screen
This is what I get with the C flag patch applied:
Welcome to Aurora SPARC Linux
+--------------+ Error +---------------+
| |
| failed to read keymap information: |
| Success |
| |
| +----+ |
| | OK | |
| +----+ |
| |
| |
+--------------------------------------+
<Tab>/<Alt-Tab> between elements | <Space> selects | <F12> next screen
- [Qemu-devel] sparc64 lazy conditional codes evaluation, Igor Kovalenko, 2010/05/03
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Blue Swirl, 2010/05/03
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Igor Kovalenko, 2010/05/03
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Blue Swirl, 2010/05/03
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Igor Kovalenko, 2010/05/03
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation,
Blue Swirl <=
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Igor Kovalenko, 2010/05/05
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Blue Swirl, 2010/05/06
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Igor Kovalenko, 2010/05/08
- [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Blue Swirl, 2010/05/09
- Re: [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Mark Cave-Ayland, 2010/05/10
- Re: [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Blue Swirl, 2010/05/10
- Re: [Qemu-devel] Re: sparc64 lazy conditional codes evaluation, Mark Cave-Ayland, 2010/05/15