[Xen-devel] Re: [patch] pae: tlbflush linear page table updates

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 12 Aug 2005, at 10:56, Andi Kleen wrote:

> > "early clobber" is good to prevent output alias as input, if that 
> > output
> > may be clobbered before input is used. However there seems no point to
> > simply alias among inputs. Or else, the only way I can see is that
> > compiler insert some extra lines in the middle of inline asm... Any
> > benefit for this likelihood?
> There can be cases e.g. when the inputs are dependent and you use
> general enough constraints. e.g. one input can be %reg and the other
> offset(%reg). In this case they will essentially alias.
That would be a nasty one to track down. :-)

It seems the general policy in Linux also is not to bother with '&' on 
outputs that have a forced alias in the input list. I'd be happy to see 
a cleanup patch that adds more '&'s to Xen though -- in cases where we 
are relying on the forced alias then it's unlikely to change the 
generated assembly code.
 -- Keir


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