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xen-devel
Re: [Xen-devel] Essay on an important Xen decision (long)
Mark Williamson wrote:
I imagine that you would have to always have shadow paging enable but
you could still do bulk updates ala writable page tables so the
performance cost should be minimal I would think.
Trying to understand the memory system in more details so any additional
info is much appreciate :-)
I don't see why that couldn't perform decently, although it'd have more
overhead than allowing the guest to manage its pagetables directly... I
*thought* this was intended to be supported at some point, but I'm not sure
if it's been needed yet. Others may have more concrete numbers for the
performance - I think writable PTs got benchmarked against shadowing at some
point.
Just to be thorough, was the shadow paging code a "pure" shadow page
table where ever PTE write trapped to the hypervisor or were bulk PMD
updates sent to the hypervisor?
I'm surprised there would be a measurable difference with shadow paging
as it should only require a potential allocation (which could be fast
pathed) and in the normal case, a couple extra reads/writes. I would
think that cost would be overshadowed by the original cost of the
context switch.
Of course, I guess it wouldn't be that much of a shock to me that the
overhead is at least measurable...
Regards,
Anthony Liguori
Cheers,
Mark
Thanks,
Anthony Liguori
If ia64 does decide to back off from the P==M route then I suspect VP
is the way to go (which is I think how ia64 domU's currently work
anyway).
-- Keir
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