Re: [PATCH v7 26/27] tcg: enable MTTCG by default for ARM on x86 hosts

["Alex Bennée" <alex.bennee@linaro.org>]

Pranith Kumar <bobby.prani@gmail.com> writes:

> Alex Bennée writes:
>
>> This enables the multi-threaded system emulation by default for ARMv7
>> and ARMv8 guests using the x86_64 TCG backend. This is because on the
>> guest side:
>>
>>   - The ARM translate.c/translate-64.c have been converted to
>>     - use MTTCG safe atomic primitives
>>     - emit the appropriate barrier ops
>>   - The ARM machine has been updated to
>>     - hold the BQL when modifying shared cross-vCPU state
>>     - defer cpu_reset to async safe work
>>
>> All the host backends support the barrier and atomic primitives but
>> need to provide same-or-better support for normal load/store
>> operations.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> <snip>
>
>>
>> +/* This defines the natural memory order supported by this
>> + * architecture before guarantees made by various barrier
>> + * instructions.
>> + *
>> + * The x86 has a pretty strong memory ordering which only really
>> + * allows for some stores to be re-ordered after loads.
>> + */
>> +#include "tcg-mo.h"
>> +
>> +static inline int get_tcg_target_mo(void)
>> +{
>> +    return TCG_MO_ALL & ~TCG_MO_LD_ST;
>> +}
>> +
>
> Shouldn't this be TCG_MO_ALL & ~TCG_MO_ST_LD?

The case that x86 doesn't handle normally is store-after-load which is
what I assumed TCG_MO_LD_ST was. Perhaps we need some better comments
for each of the enums?

>
> Thanks,


--
Alex Bennée