Re: Boot flakiness with QEMU 3.1.0 and Clang built kernels

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Apr 14, 2020 at 12:05:53PM +1000, David Gibson wrote:
> On Sat, Apr 11, 2020 at 11:57:23PM +1000, Nicholas Piggin wrote:
> > Nicholas Piggin's on April 11, 2020 7:32 pm:
> > > Nathan Chancellor's on April 11, 2020 10:53 am:
> > >> The tt.config values are needed to reproduce but I did not verify that
> > >> ONLY tt.config was needed. Other than that, no, we are just building
> > >> either pseries_defconfig or powernv_defconfig with those configs and
> > >> letting it boot up with a simple initramfs, which prints the version
> > >> string then shuts the machine down.
> > >> 
> > >> Let me know if you need any more information, cheers!
> > > 
> > > Okay I can reproduce it. Sometimes it eventually recovers after a long
> > > pause, and some keyboard input often helps it along. So that seems like 
> > > it might be a lost interrupt.
> > > 
> > > POWER8 vs POWER9 might just be a timing thing if P9 is still hanging
> > > sometimes. I wasn't able to reproduce it with defconfig+tt.config, I
> > > needed your other config with various other debug options.
> > > 
> > > Thanks for the very good report. I'll let you know what I find.
> > 
> > It looks like a qemu bug. Booting with '-d int' shows the decrementer 
> > simply stops firing at the point of the hang, even though MSR[EE]=1 and 
> > the DEC register is wrapping. Linux appears to be doing the right thing 
> > as far as I can tell (not losing interrupts).
> > 
> > This qemu patch fixes the boot hang for me. I don't know that qemu 
> > really has the right idea of "context synchronizing" as defined in the
> > powerpc architecture -- mtmsrd L=1 is not context synchronizing but that
> > does not mean it can avoid looking at exceptions until the next such
> > event. It looks like the decrementer exception goes high but the
> > execution of mtmsrd L=1 is ignoring it.
> > 
> > Prior to the Linux patch 3282a3da25b you bisected to, interrupt replay
> > code would return with an 'rfi' instruction as part of interrupt return,
> > which probably helped to get things moving along a bit. However it would
> > not be foolproof, and Cedric did say he encountered some mysterious
> > lockups under load with qemu powernv before that patch was merged, so
> > maybe it's the same issue?
> > 
> > Thanks,
> > Nick
> > 
> > The patch is a bit of a hack, but if you can run it and verify it fixes
> > your boot hang would be good.
> 
> So a bug in this handling wouldn't surprise me at all.  However a
> report against QEMU 3.1 isn't particularly useful.
> 
>  * Does the problem occur with current upstream master qemu?

Yes, I can reproduce the hang on 5.0.0-rc2.

>  * Does the problem occur with qemu-2.12 (a pretty widely deployed
>    "stable" qemu, e.g. in RHEL)?

No idea but I would assume so. I might have time later this week to test
but I assume it is kind of irrelevant if it is reproducible at ToT.

> > ---
> > 
> > diff --git a/target/ppc/translate.c b/target/ppc/translate.c
> > index b207fb5386..1d997f5c32 100644
> > --- a/target/ppc/translate.c
> > +++ b/target/ppc/translate.c
> > @@ -4364,12 +4364,21 @@ static void gen_mtmsrd(DisasContext *ctx)
> >      if (ctx->opcode & 0x00010000) {
> >          /* Special form that does not need any synchronisation */
> >          TCGv t0 = tcg_temp_new();
> > +        TCGv t1 = tcg_temp_new();
> >          tcg_gen_andi_tl(t0, cpu_gpr[rS(ctx->opcode)],
> >                          (1 << MSR_RI) | (1 << MSR_EE));
> > -        tcg_gen_andi_tl(cpu_msr, cpu_msr,
> > +        tcg_gen_andi_tl(t1, cpu_msr,
> >                          ~(target_ulong)((1 << MSR_RI) | (1 << MSR_EE)));
> > -        tcg_gen_or_tl(cpu_msr, cpu_msr, t0);
> > +        tcg_gen_or_tl(t1, t1, t0);
> > +
> > +        gen_update_nip(ctx, ctx->base.pc_next);
> > +        gen_helper_store_msr(cpu_env, t1);
> >          tcg_temp_free(t0);
> > +        tcg_temp_free(t1);
> > +        /* Must stop the translation as machine state (may have) changed */
> > +        /* Note that mtmsr is not always defined as context-synchronizing */
> > +        gen_stop_exception(ctx);
> > +
> >      } else {
> >          /*
> >           * XXX: we need to update nip before the store if we enter
> > 
> 
> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson

Cheers,
Nathan