Re: paravirtualization structure

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Omar Khan wrote:
> Dor Laor <dor.laor@...> writes:
>
>
>   
>> Theoretically speaking the answer is yes, vmcall is a special
>> instruction that it one and only purpose is the exit from the guest.
>> While mov cr3 instruction might be bound to other actions of the
>> processor and might have more effects on the pipeline, this harder to
>> optimize for vmexit purposes.
>>
>>     
>
> I guess what is confusing me is that the vmcall causes an vmexit in VMX non-root
> mode (the one in which a paravirtualized guest is operating), but Avi says that
>  a hypercal (made using a vmcall) 'avoids' a vmentry/vmexit and Ingo comments in
> his patch that :
>
> "This utilizes the cr3 filter capability of the hardware - if this works out
> then no VM exit happens"
>
> NO VM-exits happen. hmmmm... maybe I need to go over the intel manuals more
> thoroughly, however any hints will be most welcomed. 
>   

Using the cr3 filter does not involve a hypercall.  Instead, it allows 
the guest to context switch without trapping on the mov %cr3 
instruction, by providing a set of pre-prepared cr3s.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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