Re: [Xen-devel] [PATCH RFC 00/12] Nested p2m: allow sharing between vCPUs

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

On Mon, 2017-08-28 at 18:03 +0100, George Dunlap wrote:
> On 07/18/2017 11:34 AM, Sergey Dyasli wrote:
> > Nested p2m (shadow EPT) is an object that stores memory address
> > translations from L2 GPA directly to L0 HPA. This is achieved by
> > combining together L1 EPT tables with L0 EPT during L2 EPT violations.
> > 
> > In the usual case, L1 uses the same EPTP value in VMCS12 for all vCPUs
> > of a L2 guest. But unfortunately, in current Xen's implementation, each
> > vCPU has its own n2pm object which cannot be shared with other vCPUs.
> > This leads to the following issues if a nested guest has SMP:
> > 
> >     1. There will be multiple np2m objects (1 per nested vCPU) with
> >        the same np2m_base (L1 EPTP value in VMCS12).
> > 
> >     2. Same EPT violations will be processed independently by each vCPU.
> > 
> >     3. Since MAX_NESTEDP2M is defined as 10, if a domain has more than
> >        10 nested vCPUs, performance will be extremely degraded due to
> >        constant np2m LRU list thrashing and np2m flushing.
> > 
> > This patch series makes it possible to share one np2m object between
> > different vCPUs that have the same np2m_base. Sharing of np2m objects
> > improves scalability of a domain from 10 nested vCPUs to 10 nested
> > guests (with arbitrary number of vCPUs per guest).
> 
> On the whole this looks like a decent approach.
> 
> Were you planning on re-sending it with the RFC removed, or would you
> like me to do a detailed review of this series as it is?

Thanks for review! My current plan is to re-send the series as v1 after
addressing your and Christoph's comments. Let's wait for that before
detailed review :)

Oh and there is a possibility I may do some AMD testing.

-- 
Thanks,
Sergey
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