Re: [Qemu-devel] Status of my hacks on the MTTCG WIP branch

[alvise rigo]

On Fri, Jan 15, 2016 at 3:51 PM, Alex Bennée <address@hidden> wrote:
>
> alvise rigo <address@hidden> writes:
>
>> This problem could be related to a missing multi-threaded aware
>> translation of the atomic instructions.
>> I'm working on this missing piece, probably the next week I will
>> publish something.
>
> Maybe. We still have Fred's:
>
>   Use atomic cmpxchg to atomically check the exclusive value in a STREX
>
> Which I think papers over the cracks for both arm and aarch64 in MTTCG
> while not being as correct as your work.

Keep in mind that Linux on arm64 uses the LDXP/STXP instructions that
exist solely in aarch64.
These instructions are purely emulated now and can potentially write
128 bits of data in a non-atomic fashion.

Regards,
alvise

>
>>
>> Regards,
>> alvise
>>
>> On Fri, Jan 15, 2016 at 3:24 PM, Pranith Kumar <address@hidden> wrote:
>>> Hi Alex,
>>>
>>> On Fri, Jan 15, 2016 at 8:53 AM, Alex Bennée <address@hidden> wrote:
>>>> Can you try this branch:
>>>>
>>>>
>>>> https://github.com/stsquad/qemu/tree/mttcg/multi_tcg_v8_wip_ajb_fix_locks-r1
>>>>
>>>> I think I've caught all the things likely to screw up addressing.
>>>>
>>>
>>> I tried this branch and the boot hangs like follows:
>>>
>>> [    2.001083] random: systemd-udevd urandom read with 1 bits of entropy
>>> available
>>> main-loop: WARNING: I/O thread spun for 1000 iterations
>>> [   23.778970] INFO: rcu_sched detected stalls on CPUs/tasks: {} (detected
>>> by 0, t=2102 jiffies, g=-165, c=-166, q=83)
>
> This is just saying the kernel has been waiting for a while and nothing
> has happened.
>
>>> I will try to debug and see where it is hanging.
>
> If we knew what the kernel was waiting for that would be useful to know.
>
>>>
>>> Thanks!
>>> --
>>> Pranith
>
>
> --
> Alex Bennée