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Re: [Qemu-devel] [PATCH 6/8] tcg/tci: disable MTTCG if TCI is enabled


From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 6/8] tcg/tci: disable MTTCG if TCI is enabled
Date: Thu, 3 Aug 2017 10:56:03 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 08/03/2017 10:40 AM, Philippe Mathieu-Daudé wrote:
> On 07/26/2017 05:02 AM, Alex Bennée wrote:
>>
>> Philippe Mathieu-Daudé <address@hidden> writes:
>>
>>> Hi Peter,
>>>
>>> I think this patch belongs to 2.10, as there is no time to fix TCI +
>>> MTTCG. Should I RESEND it alone with "for 2.10" subject?
>>>
>>> One other option might be disable TCI if MTTCG enabled, but there is
>>> no ./configure option for MTTCG while there is for TCI.
>>>
>>> Regards,
>>>
>>> Phil.
>>>
>>> On 06/28/2017 10:02 PM, Philippe Mathieu-Daudé wrote:
>>>> TCI + MTTCG cause strange errors...
>>>>
>>>>     $ arm-softmmu/qemu-system-arm -machine raspi2 -cpu cortex-a7 -smp 4
>>>> -accel tcg,thread=multi -kernel kernel7.img
>>>>     qemu-system-arm: Guest expects a stronger memory ordering than the host
>>>> provides
>>>>     This may cause strange/hard to debug errors
>>>>     Segmentation fault (core dumped)
>>
>> So this isn't TCI enabling MTTCG by accident - this is the user forcing
>> it when it wouldn't otherwise be enabled. Hence the scary warning...
> 
> Hmmm this is the user enabling TCI, I'm not sure about "forcing it".
> Now MTTCG is enforced by the ./configure and there is no option to disable it.

Er, no.

If you write "-accel tcg" you are not forcing mttcg on.
If you write "-accel tcg,thread=multi" you are forcing it on.
If you write "-accel tcg,thread=single" you are forcing it off.

MTTCG is only enabled by default if TCG_TARGET_DEFAULT_MO is defined, and in
the case of TCI, it is not.


r~



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