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Re: [Xen-devel] [PATCH - proposed] XI Shadow Page Table Mechanism]

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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