[Qemu-devel] Semantics of tcg_enabled() and what it means for multi-arch

[Peter Crosthwaite]

Hi Paolo, Richard and all,

What is the exact semantic of tcg_enabled() supposed to be and is it
ill-defined in multi-arch?

Currently, tcg_enabled is defined as:

bool tcg_enabled(void)
{
    return tcg_ctx.code_gen_buffer != NULL;
}

In the multi-arch work, the tcg_ctx is now multiple per-arch. So lets
assume that we virtualise tcg_enabled as a CPU hook. This handles a
good number of cases, where there is a sense of a current CPU to which
the tcg_enabled is being queried. All uses in target-foo are trivially
handled as they will link against their local tcg_enabled
implementation. This per-cpu approach has the added advantage of
preparing support for mixed KVM/TCG multi-arch systems.

But tcg_enabled() is used in the memory API for dirty code tracking
where there is no sense of a current tcg_ctx:

include/exec/ram_addr.h:                if (tcg_enabled()) {
include/exec/ram_addr.h:        uint8_t clients = tcg_enabled() ?
DIRTY_CLIENTS_ALL : DIRTY_CLIENTS_NOCODE;
memory.c:    mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
memory.c:    mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
memory.c:    mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
memory.c:    mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;

So what is the correct logic for populating dirty_log_mask and friends
when there are 0. 1, or more TCG engines?

Regards,
Peter