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

[zhu <vanbas.han@xxxxxxxxx>]

Hi,
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,
hanzhu



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