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


From: Philippe Mathieu-Daudé
Subject: Re: [Qemu-devel] [PATCH 6/8] tcg/tci: disable MTTCG if TCI is enabled
Date: Thu, 3 Aug 2017 15:04:47 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 08/03/2017 02:56 PM, Richard Henderson wrote:
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.

Ok! I obviously missed this part.

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.

Ok, so I misunderstood Alex :) I thought about ./configure enabled while he was describing -accel enabled.

Thanks for pointing that out :)



r~




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