linux-kernel - Re: use of setjmp/longjmp in x86 emulator.

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Message-ID: <20100302072809.GS16909@redhat.com>
Date:	Tue, 2 Mar 2010 09:28:09 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	john cooper <john.cooper@...rd-harmonic.com>
Cc:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, avi@...hat.com,
	mtosatti@...hat.com, zamsden@...hat.com
Subject: Re: use of setjmp/longjmp in x86 emulator.

On Mon, Mar 01, 2010 at 02:13:32PM -0500, john cooper wrote:
> Gleb Natapov wrote:
> 
> >Think about what happens if in the middle of
> >instruction emulation some data from device emulated in userspace is
> >needed. Emulator should be able to tell KVM that exit to userspace is
> >needed and restart instruction emulation when data is available.
> 
> setjmp/longjmp are useful constructs in general but
> IME are better suited for infrequent exceptions vs.
> routine usage.
Exception condition during instruction emulation _is_
infrequent. Although setjmp/longjmp that I know about
are routine usage. See QEMU TCG main loop or userspace
thread libraries.

> If the issue is finding some clean and regular way
> to back out from (and possibly reeneter) logic
> expressed within nested function invocations, have
> you considered turning the problem inside out and
> using a state machine approach?
I don't see how state machine will help. But the goal
is not to rewrite emulator.c (this will no be excepted
by kvm maintainers), but improve it gradually.

--
			Gleb.
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