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[Xen-devel] Re: Page fault is 4 times faster with XI shadow mechanism

To: Robert Phillips <rsp.vi.xen@xxxxxxxxx>
Subject: [Xen-devel] Re: Page fault is 4 times faster with XI shadow mechanism
From: zhu <vanbas.han@xxxxxxxxx>
Date: Sun, 02 Jul 2006 12:20:49 +0800
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Robert Phillips д:
Well I don't.  I simply pre-allocate a pool of SPTI's.  It can be quite a
large pool but certainly not one-SPTI per MFN.  SPTIs are allocated on
demand (when a guest page needs to be shadowed) and, when the pool runs low,
the LRU SPTs are torn down and their SPTIs recycled.

Well what I mean is that we should not connect a snapshot page with a SPTI at the first time the SPTIs are reserved. It would be better to manage these snapshot pages in another dynamic pool.
BTW: What do you think of the backlink issue mentioned in my previous mail?
Currently I allocate about 5% of system memory for this purpose (this
includes the SPT, its snapshot and the backlink pages) and, with that
reasonable investment, we get very good performance.  With more study, I'm
sure things could be tuned even better.  (I hope I have properly understood
your questions.)

-- rsp

On 7/1/06, zhu <vanbas.han@xxxxxxxxx > wrote:

After taking some time to dig into your patch about XI Shadow page
table, I have to say it's really a good design and implementation IMHO,
especially the parts about the clear hierarchy for each smfn,decision
table and how to support 32nopae in a rather elegant way. However, I
have several questions to discuss with you.:-)
1) It seems XI shadow pgt reserve all of the possible resources at the
early stage for HVM domain(the first time to create the asi). It could
be quite proper to reserve the smfns and sptis. However, do we really
need to reserve one snapshot page for each smfn at first and retain it
until the HVM domain is destroyed? I guess a large number of gpts will
not been modified frequently after them are totally set up. IMHO, it
would be better to manage these snapshot pages dynamic. Of course, this
will change the basic logistic of the code, e.g. you have to sync the
shadow pgt when invoke spti_make_shadow instead of leaving it out of
sync, you can't set up the total low level shadow pgt when invoke
resync_spte  since it could cost a lot of time.
2) GP back link plays a very important role in XI shadow pgt. However,
it will also cause high memory pressure for the domain(2 pages for each
smfn). For these normal guest pages instead of GPT pages, I guess its
usage is limited. Only when invoke xi_invld_mfn, divide_large_page or
dirty logging, we will refer to the back link for these normal guest
pages. Is it reasonable to implement the back link only for the GPT
pages? Of course, this will increase the complexity of the code a little.
3) Can you show us the statistics between the current shadow pgt and XI
pgt for some critical operations, such as shadow_resync_all, gva_to_gpa,
shadow_fault and so on. I'm really curious about it.

I have to say I'm not very familiar with the current shadow pgt
implementation so I could miss some important considerations when I post
these questions. Please point it out.
Thanks for sharing your idea and code with us. :-)

Best Regards,

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