/*
* We default to false if we know other options have been enabled
* which are currently incompatible with MTTCG. Otherwise when each
* guest (target) has been updated to support:
* - atomic instructions
* - memory ordering primitives (barriers)
* they can set the appropriate CONFIG flags in ${target}-softmmu.mak
*
* Once a guest architecture has been converted to the new primitives
* there are two remaining limitations to check.
*
* - The guest can't be oversized (e.g. 64 bit guest on 32 bit host)
* - The host must have a stronger memory order than the guest
*
* It may be possible in future to support strong guests on weak hosts
* but that will require tagging all load/stores in a guest with their
* implicit memory order requirements which would likely slow things
* down a lot.
*/
static bool check_tcg_memory_orders_compatible(void)
{
#if defined(TCG_GUEST_DEFAULT_MO) && defined(TCG_TARGET_DEFAULT_MO)
return (TCG_GUEST_DEFAULT_MO & ~TCG_TARGET_DEFAULT_MO) == 0;
#else
return false;
#endif
}That function will return false for x86_64 on ARM64.
On tcg/aarch64/tcg-target.h
#define TCG_TARGET_DEFAULT_MO (0)
On target/i386/cpu.h
/* The x86 has a strong memory model with some store-after-load re-ordering */
#define TCG_GUEST_DEFAULT_MO (TCG_MO_ALL & ~TCG_MO_ST_LD)
I am asking for clarification because the above code will default x86 on ARM64 to disable MTTCG.