Re: [Qemu-devel] exec: About DISAS_JUMP and DISAS_UPDATE

From: Richard Henderson <rth@twiddle.net>
To: Sergey Fedorov <serge.fdrv@gmail.com>, qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Peter Crosthwaite <crosthwaite.peter@gmail.com>,
	Aurelien Jarno <aurelien@aurel32.net>,
	Peter Maydell <peter.maydell@linaro.org>
Subject: Re: [Qemu-devel] exec: About DISAS_JUMP and DISAS_UPDATE
Date: Thu, 22 Oct 2015 09:50:41 -1000	[thread overview]
Message-ID: <56293E11.3080206@twiddle.net> (raw)
In-Reply-To: <56292AEA.3010408@gmail.com>

On 10/22/2015 08:28 AM, Sergey Fedorov wrote:
> Hi all,
>
> I am trying to understand what the difference should be between
> DISAS_JUMP and DISAS_UPDATE. Actually, these macros have comments in
> include/exec/exec-all.h which say that DISAS_JUMP should be used when
> only PC was modified dynamically whereas DISAS_UPDATE should be used
> when some other CPU state was (in addition to PC?) modified dynamically.
> In fact, every target except ARM AArch64 does not distinguish between
> them. As I can see ARM AArch64 seems to suppose that: (1) PC was not
> modified when DISAS_UPDATE is used and should be updated with dc->pc
> when finishing translation; (2) DISAS_JUMP can be used to indicate that
> a new PC value was set and it should be preserved when finishing
> translation.
>
> So I'm a bit confused... What the difference should be? Maybe something
> should be fixed/clarified to make the comments and the code consistent.

It's a mistake that these are defined in exec/.  They ought to be totally 
private to each translator.  See e.g. ExitStatus in target-alpha/translate.c.

But yes, what you see in aarch64 is approximately what is intended.

r~