Re: [Qemu-devel] [Qemu-discuss] ppc and icount

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jan 10, 2018 at 10:34:18AM +0000, Peter Maydell wrote:
> On 10 January 2018 at 08:57, Steven Seeger
> <steven.seeger@flightsystems.net> wrote:
> > Sorry for another post. I did a bisect and found what is the bad commit for
> > me:
> >
> > 044897ef4a22af89aecb8df509477beba0a2e0ce is the first bad commit
> > commit 044897ef4a22af89aecb8df509477beba0a2e0ce
> > Author: Richard Purdie <richard.purdie@linuxfoundation.org>
> > Date:   Mon Dec 4 22:25:43 2017 +0000
> >
> >     target/ppc: Fix system lockups caused by interrupt_request state
> > corruption
> 
> Great -- thanks for the bisect. Let's take this to the -devel list;
> I've cc'd the PPC maintainers.
> 
> Context: Steven reports that we broke -icount for PPC guests with
> this commit:
> 
> $ ./build/all/ppc-softmmu/qemu-system-ppc  -icount auto
> qemu: fatal: Raised interrupt while not in I/O function
> NIP fff08978   LR fff08904 CTR 00000000 XER 00000000 CPU#0
> MSR 00000000 HID0 00000000  HF 00000000 iidx 3 didx 3
> Bad icount read
> 
> The backtrace from the assert is:
> 
> #0  tcg_handle_interrupt (cpu=0x7ffff7fc2010, mask=4) at qemu/accel/tcg/tcg-
> all.c:58
> #1  0x0000555555962aa4 in cpu_interrupt (cpu=0x7ffff7fc2010, mask=4) at qemu/
> include/qom/cpu.h:859
> #2  0x0000555555962e55 in cpu_interrupt_exittb (cs=0x7ffff7fc2010) at qemu/
> target/ppc/helper_regs.h:105
> #3  0x0000555555964505 in do_rfi (env=0x7ffff7fca2b0, nip=197460, msr=4096)
>     at qemu/target/ppc/excp_helper.c:998
> #4  0x0000555555964555 in helper_rfi (env=0x7ffff7fca2b0) at qemu/target/ppc/
> excp_helper.c:1008
> #5  0x00007fffe7c124b9 in code_gen_buffer ()
> 
> The problem is that icount was relying on the previous
> handling of do_rfi() as "just set state as we know we're
> going to be last insn in the TB".
> 
> Not sure how best to fix this (mark the insn as IO ok?)

Aw, man.  I've become target-ppc tcg maintainer by default, but tbh my
knowledge wasn't really deep enough to understand the problem that
044897ef was fixing in the first place.  And I barely know what icount
does at all.

-- 
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

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