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

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Message-ID: <4B8C4F12.8050009@redhat.com>
Date:	Mon, 01 Mar 2010 13:34:42 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, avi@...hat.com, mtosatti@...hat.com
Subject: Re: use of setjmp/longjmp in x86 emulator.

On 03/01/2010 12:56 PM, H. Peter Anvin wrote:
> On 03/01/2010 02:31 PM, H. Peter Anvin wrote:
>    
>> On 03/01/2010 11:18 AM, Zachary Amsden wrote:
>>      
>>> It's going to be ugly to emulate segmentation, NX and write protect
>>> support without hardware to do this checking for you, but it's just what
>>> you have to do in this slow path - tedious, fully specified emulation.
>>>
>>> Just because it's tedious doesn't mean we need to use setjmp / longjmp.
>>> Throw / catch might be effective, but it's still pretty bizarre to do
>>> tricks like that in C.
>>>
>>>        
>> Well, setjmp/longjmp really is not much more than exception handling in C.
>>
>>      
> For what it's worth, I think that setjmp/longjmp is not anywhere near as
> dangerous as people want to make it out to be.  gcc will warn for
> dangerous uses (and a lot of non-dangerous uses), but generally the
> difficult problems can be dealt with by moving the setjmp-protected code
> into a separate function.
>    

I'd be curious to see if it would need to evolve it to preemptsetjmp / 
irqlongjmp or some other more complex forms in time.

But I'd rather implement a new language where acquisition of resources 
such as locks, dynamically allocated objects, and ref counts are 
predicated in the function typing and are heavily encouraged to possess 
defined inverses.  Then the closure of a particular layer of nesting 
already has enough information to provide release upon escape, and the 
compiler can easily take the burden of checking for a large class of 
lock and resource violation.

And it would have to be prettier than the current languages that do 
that, meaning operator overloading would be banned.  Although it would 
define rational numbers, super-extended precision arithmetic, imaginary 
numbers, quaternions and matrices as part of the spec, so there would be 
no need to use arithmetic overrides anyway, and then all the nonsensical 
operators could die, die, die, especially the function () and logical 
operator overrides.

Zach
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