Re: [Xen-merge] arch/i386/mach-xen/mm/highmem.c indentical

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 9 Aug 2005, at 18:27, Andi Kleen wrote:

> >  2. queue_l1_update. This is best used for singleton updates. The
> > 'queue' in the name is a bit of a misnomer as we now no longer ever
> > queue things because: (a) working out where to put the flushes to 
> > avoid
> > read-after-write hazards on pending updates is hard, and  (b) its
> > impossible to make SMP guests work with good perofrmance. [This is one
> Can you explain the SMP guest problem in more detail?
The queued interface as it stands cannot work correctly for SMP guests 
because it provides no atomic read-modify-write command. So you cannot 
atomically toggle pte flags or xchg with zero, or any of the other 
atomic things you need to do for safety on SMP. We could add some 
atomic commands, but our solution was just to move over to writable 
pagetables which have a simpler interface for guests and good batching 
performance.
 -- Keir


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