Re: KVM & VMX root mode

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

Sander van Leeuwen wrote:
> Avi Kivity wrote:
>> Sander van Leeuwen wrote:
>>   
>>> You are mistaken. VT-x allows multiple clients to work concurrently.
>>> There's only one catch: all those
>>> clients should follow certain guidelines.
>>>     
>>
>> Currently, kvm assumes that it is the only user that issues vmptrld
>> instructions.  There may me other such assumptions in the code.
>>
>> There is also the question of who issues the vmxon/vmxoff instructions
>> and set the magic msr bits that enable kvm.  You can't have two
>> hypervisors doing that; that would be racy.  It should be possible to
>> abstract those bits out to something outside kvm, but I don't see that
>> happening.
>>
>>   
> VirtualBox currently always executes vmxon when it wants to execute 
> some guest code. When leaving
> ring 0 it issues the corresponding vmxoff. While that may increase 
> world switch overhead, it has not turned
> out to be an issue for us. 

The overhead will increase with newer hardware, as more state is cached 
on-chip.  It's probably okay when deploying to a desktop but i don't 
think it is good enough for a server.

> I think it's a bit risky to use somebody else's vmxon pointer.
>
> You have a point with the msr updates though. 

I think the easiest way forward is for you to unload kvm-intel (or 
kvm-amd) when you load, and to reload it when you unload.

-- 
error compiling committee.c: too many arguments to function


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