Re: [PATCH] SVM: forbid guest to execute monitor/mwait

["Joerg Roedel" <joerg.roedel-5C7GfCeVMHo@public.gmane.org>]

On Thu, Mar 22, 2007 at 02:01:18PM +0200, Avi Kivity wrote:
> Joerg Roedel wrote:
> >On Thu, Mar 22, 2007 at 01:40:26PM +0200, Avi Kivity wrote:
> >  
> >>Joerg Roedel wrote:
> >>    
> >>>Right. But it is possible for userspace to enable monitor bit for the
> >>>guest. Without virtualization of these 2 instructions the guest would
> >>>idle in the guest state after calling mwait and prevent other processes
> >>>and guests from running in that time.       
> >>Won't an interrupt during mwait cause a vmexit?
> >>    
> >
> >Yes, an interrupt would cause an vmexit. But depending on the HZ value
> >of the host it may be a lot of time until the next interrupt. And over
> >time this will sum up and slow down other tasks.
> >  
> 
> Well, a guest can cause exactly the same effect with
> 
>    cli
> spin:
>    jmp spin
> 
> without executing any instructions that are not reported by cpuid.

Intentionally, yes. In this case the guest knows what it is doing and we
have no way to prevent such things anyway. If it executes mwait this
delay loop is not intended by the guest.

To summarize my point:

We agree that we should hide the monitor cpuid bit to the guest from the
kernel level. Allowing the guest to use mwait if its available on the
processor without proper virtualization would cause the processor to
idle in the guest mode.
I would additionally forbid the guest to execute these instructions.
Likely the guest won't use them anyway when the cpuid bit is not set,
but if this bit is not set, the execution of these instructions should
cause an #UD as in any real processor, imho.

Joerg

-- 
Joerg Roedel
Operating System Research Center
AMD Saxony LLC & Co. KG



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