Re: [patch] SMP alternatives

[Jeff V. Merkey]

Linus Torvalds wrote:

> On Wed, 23 Nov 2005, H. Peter Anvin wrote:
>  
>
> > Linus Torvalds wrote:
> >    
> >
> > > What I suggested to Intel at the Developer Days is to have a MSR (or, better
> > > yet, a bit in the page table pointer %cr0) that disables "lock" in _user_
> > > space. Ie a lock would be a no-op when in CPL3, and only with certain
> > > processes.
> > >      
> > You mean %cr3, right?
> >    
>
> Yes. 
>
> It _should_ be fairly easy to do something like that - just a simple 
> global flag that gets set and makes CPL3 ignore lock prefixes. Even timing 
> doesn't matter - it it takes a hundred cycles for the setting to take 
> effect, we don't care, since you can't write %cr3 from user space anyway, 
> and it will certainly take a hundred cycles (and a few serializing 
> instructions) until we get to CPL3.
>
> I'd personally prefer it to be in %cr3, since we'd have to reload it on 
> task switching, and that's one of the registers we load anyway. And it 
> would make sense. But it could be in an MSR too.
>
> Of course, if it's in one of the low 12 bits of %cr3, there would have to 
> be a "enable this bit" in %cr4 or something. Historically, you could write 
> any crap in the low bits, I think.
>
> 		Linus
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>  
The lock prefix '0F' is used for a lot of opcodes other than "lock". Go 
check the instruction set reference. It's not
trivial what you are proposing. Intel has a pretty hacked up opcode map 
with a lot of history. The bit should be in
CR4 and not CR3.

J
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