Re: [PATCH 0 of 5] v2: Nested-p2m cleanups and locking changes

[Christoph Egger <Christoph.Egger@amd.com>]

On 06/27/11 17:48, 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
>>>>     happen.
>>>
>>> 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.

This patch is great! It gives the speed up the XP mode needs to continue 
booting. We talked about this at the Xen-Hackathon you remember?

> 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.

Ok.

Christoph


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