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

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Message-ID: <4B936B27.1050207@redhat.com>
Date:	Sun, 07 Mar 2010 11:00:23 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	john cooper <john.cooper@...rd-harmonic.com>,
	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, mtosatti@...hat.com,
	zamsden@...hat.com
Subject: Re: use of setjmp/longjmp in x86 emulator.

On 03/02/2010 09:28 AM, Gleb Natapov wrote:
> 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.

Well, with mmio you'd expect it to happen every read access.

> Although setjmp/longjmp that I know about
> are routine usage. See QEMU TCG main loop or userspace
> thread libraries.
>    

Agreed, nothing magical about it.

>> 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.
>    

That is orthogonal.  If we decide a state machine is the best 
implementation, then we'll find a way to move over to that.  However, I 
don't think a state machine is a good representation considering some of 
the code paths are very complicated and depend on a many memory accesses 
(e.g. hardware task switches).

-- 
error compiling committee.c: too many arguments to function

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