On Sun, Jul 11, 2010, Avi Kivity wrote about "Re: [PATCH 0/24] Nested VMX, v5":
quoted text
> >nesting-
> >aware L1 guest hypervisors to actually use that internal structure to 
> >modify
> >vmcs12 directly, without vmread/vmwrite and exits.
> >   
> 
> No, they can't, since (for writes) L0 might cache the information and 
> not read it again.  For reads, L0 might choose to update vmcs12 on demand.

Well, in the current version of the nested code, all L0 does on a L1 vmwrite
is to update the in-memory vmcs12 structure. It doesn't not update vmcs02,
nor cache anything, nor remember what has changed and what hasn't. So replacing
it with a direct write to the memory structure should be fine...

Of course, this situation isn't optimal, and we *should* optimize the number of
unnecessary vmwrites L2 entry and exit (and we actually tried some of this
in our tech report), but it's not in the current patch set.  When we do these
kind of optimizations, you're right that:

quoted text
> A pvvmread/write needs to communicate with L0 about what fields are 
> valid (likely using available and dirty bitmaps).


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Nadav Har'El                        |           Sunday, Jul 11 2010, 1 Av 5770
nyh@math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |If marriage was illegal, only outlaws
http://nadav.harel.org.il           |would have in-laws.
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