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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Thu, 31 Mar 2016 21:56:50 +0200
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Subject: Re: [Qemu-devel] tcg: reworking tb_invalidated_flag
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To: Sergey Fedorov <serge.fdrv@gmail.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>
Cc: Richard Henderson <rth@twiddle.net>, QEMU Developers <qemu-devel@nongnu.org>, Sergey Fedorov <sergey.fedorov@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>



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