Re: [Qemu-devel] tcg: reworking tb_invalidated_flag

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 31/03/16 17:06, Sergey Fedorov wrote:
> It should be safe to invalidate a TB while some other CPU is executing
> its translated code.

Probably it's not safe to invalidate a TB while some other thread is
executing the translated code. Direct jumps to the TB being invalidated
should be reset. In case of using direct jump method, native jump
instruction should be patched in the translated code. There are some
restrictions on modification of concurrently executing code, e.g. see
section "3.4 Atomic Modification of Machine-Code Instructions" in [1].
For instance, only aligned, 8-byte atomic code modification are safe on
AMD processors, otherwise we can wind up executing a corrupted
instruction stream. I can't see i386 TCG backend does some alignment of
the jump target when translating goto_tb TCG op. I suspect other TCG
targets also have their limitations.

Looks like we have to ensure all vCPUs are out of translated code when
doing TB patching either doing tb_add_jump() or tb_phys_invalidate().
Did I missed something?

[1]
http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/37204.pdf

Kind regards,
Sergey