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Re: [Qemu-devel] [RFC] translate-all: protect code_gen_buffer with RCU

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [RFC] translate-all: protect code_gen_buffer with RCU
Date: Sat, 23 Apr 2016 23:20:08 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

On Fri, Apr 22, 2016 at 15:41:13 +0100, Alex Bennée wrote:
> Emilio G. Cota <address@hidden> writes:
> > Known issues:
> > - Basically compile-tested only, since I've only run this with
> >   single-threaded TCG; I also tried running it with linux-user,
> >   but in order to trigger tb_flush I had to make code_gen_buffer
> >   so small that the CPU calling tb_flush would immediately fill
> >   the 2nd buffer, triggering the assert. If you have a working
> >   multi-threaded workload that would be good to test this, please
> >   let me know.
> With my latest mttcg unit tests:
> ./arm-softmmu/qemu-system-arm -machine virt,accel=tcg -cpu cortex-a15 \
>   -device virtio-serial-device -device virtconsole,chardev=ctd \
>   -chardev testdev,id=ctd -display none -serial stdio \
>   -kernel arm/tcg-test.flat -smp 4 -tcg mttcg=on \
>   -append "tight smc irq mod=1 rounds=100000"  -name arm,debug-threads=on

This is useful. Never mind the need for testing linux-user, I can test
both code paths (i.e. dynamic allocation and static buf) with qemu-system
by simply defining USE_STATIC_CODE_GEN_BUFFER.

After applying a modified version of this patch (that I'll send in
a jiffy) to your enable-mttcg-for-armv7-v1 branch (reverting first
"translate-all: introduces tb_flush_safe"), I can easily trigger
this error when setting a low enough TB size, e.g. -tb-size 32:

 CPU1: online and setting up with pattern 0xa0b78cbf
 CPU2: online and setting up with pattern 0x22287c45
 CPU3: online and setting up with pattern 0x6262c5c5
 CPU0: online and setting up with pattern 0xa65e7ad6
 qemu: flush code_size=10622184 nb_tbs=83886 avg_tb_size=126
 qemu: flush code_size=10469016 nb_tbs=83886 avg_tb_size=124
 qemu: flush code_size=10492920 nb_tbs=83886 avg_tb_size=125
 qemu: flush code_size=10477464 nb_tbs=83886 avg_tb_size=124
 qemu: flush code_size=10495800 nb_tbs=83886 avg_tb_size=125
 PASS: smc: irq: 0 errors, IRQs not checked
 Unhandled exception 3 (pabt)
 Exception frame registers:
 pc : [<e59f2028>]    lr : [<40010700>]    psr: a0000153
 sp : 400ac5c0  ip : 400ab4e8  fp : 40032ca8
 r10: 00000000  r9 : 00000000  r8 : 00000000
 r7 : 00000000  r6 : 00000000  r5 : 00000000  r4 : 00000000
 r3 : 00000000  r2 : 00000000  r1 : e59f2028  r0 : 00000000
 Flags: NzCv  IRQs on  FIQs off  Mode SVC_32
 Control: 00c5107d  Table: 40060000  DAC: 00000000
 IFAR: e59f2028    IFSR: 00000205

Any input on where to look would be appreciated. Thanks,


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