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Re: [Xen-devel] [PATCH 0 of 5] v2: Nested-p2m cleanups and locking chang

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 0 of 5] v2: Nested-p2m cleanups and locking changes
From: Christoph Egger <Christoph.Egger@xxxxxxx>
Date: Fri, 1 Jul 2011 12:00:15 +0200
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On 06/30/11 11:49, Tim Deegan wrote:
At 16:48 +0100 on 27 Jun (1309193311), Tim Deegan wrote:
At 14:15 +0100 on 27 Jun (1309184128), Tim Deegan wrote:
At 14:23 +0200 on 27 Jun (1309184586), Christoph Egger wrote:
  - Why is there a 10x increase in IPIs after this series?  I don't see
    what sequence of events sets the relevant cpumask bits to make this

In patch 1 the code that sends the IPIs was outside of the loop and
moved into the loop.

Well, yes, but I don't see what that causes 10x IPIs, unless the vcpus
are burning through np2m tables very quickly indeed.  Maybe removing the
extra flushes for TLB control will do the trick.  I'll make a patch...

I think I get it - it's a race between p2m_flush_nestedp2m() on one CPU
flushing all the nested P2M tables and a VCPU on another CPU repeatedly
getting fresh ones.  Try the attached patch, which should cut back the
major source of p2m_flush_nestedp2m() calls.

Writing it, I realised that after my locking fix, p2m_flush_nestedp2m()
isn't safe because it can run in parallel with p2m_get_nestedp2m, which
reorders the array it walks.  I'll have to make the LRU-fu independent
of the array order; should be easy enough but I'll hold off committing
the current series until I've done it.

I've just pushed 23633 - 26369, which is this series plus the change to
the LRU code (and a fix to the NULL deref you reported is folded in).
Hopefully that puts nested SVM back in at least as good a state as it
was before my locking-order patch broke it! :)

I run some tests and nested SVM is now in an even better state as
it was before. Performance is a lot better now, particularly MMIO performance. The e1000 driver needed several minutes to read the
e1000 mac address before and now it takes less than a second.


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