>   1. Xen marks the whole page as being of pgd-type, and so it can't
quoted text
>      have non-pgd contents (as the page would be RO, and any contents
>      would be validated as pgd entries).  However I can fix that by

That sounds more like a Xen bug. Why should it validate anything except
the first four entries which are used by the hardware? 

I can imagine that false sharing would be slow because you would
have unnecessary traps, but it shouldn't be a correctness issue.

quoted text
>      maintaining a separate per-cpu pgd page, and just copy the four
>      entries over when cr3 is reloaded.  This would move the
>      Xen-specific requirements into the Xen code without affecting the
>      rest of the kernel.

x86-64 used to do that, but it turned out this breaks some shared cache 
optimizations on the P4 between SMT threads. So you might actually see 
user space performance regressions from it.

-Andi
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