Re: use of setjmp/longjmp in x86 emulator.

[Avi Kivity]

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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