Re: [Qemu-devel] tcg: reworking tb_invalidated_flag

[Paolo Bonzini]

On 31/03/2016 21:03, Sergey Fedorov wrote:
> 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?

Almost all TCG targets have naturally aligned instructions, so that's
not a problem; we can assume that 32-bit writes are atomic, though
perhaps we can change them to atomic_set just to be safe.

Only s390 and x86 can have unaligned instructions.  For x86 I suppose
you can use 1 to 3 byte nops so that the first byte of the jump ends up
at ip%4=3.  For s390 you can do the same, I don't know the encoding of
the canonical nop but an "or 0,0" instruction can do and is 16 bits wide
(in this case instructions are 16-bit aligned so you'd want ip%4=2).

Paolo