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Subject: Re: [PATCH v2 6/6] Describe KVM_MSR_STEAL_TIME
From: Glauber Costa <glommer@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, aliguori@us.ibm.com
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Organization: Red Hat
Date: Tue, 01 Feb 2011 13:54:40 -0200
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On Sun, 2011-01-30 at 15:19 +0200, Avi Kivity wrote:
> On 01/28/2011 09:52 PM, Glauber Costa wrote:
> > This patch adds documentation about usage of the newly
> > introduced KVM_MSR_STEAL_TIME.
> >
> >
> > +
> > +MSR_KVM_STEAL_TIME: 0x4b564d03
> > +
> > +	data: 4-byte alignment physical address of a memory area which must be
> > +	in guest RAM, plus an enable bit in bit 0.
> 
> 64-byte aligned:
> - avoids wrapping around a page boundary, which may let us optimize 
> things later on (see kvm_write_guest_cached()).
> - gives us 5 more unused bits to enable more options
> 
> >   This memory is expected to
> > +	hold a copy of the following structure:
> > +
> > +	struct kvm_steal_time {
> > +	  	__u64 steal;
> > + 		__u32 version;
> > + 		__u32 flags;
> > +	 	__u32 pad[6];
> > +	}
> > +
> > +	whose data will be filled in by the hypervisor periodically. Only one
> > +	write, or registration, is needed for each VCPU. The interval between
> > +	updates of this structure is arbitrary and implementation-dependent.
> > +	The hypervisor may update this structure at any time it sees fit until
> > +	anything with bit0 == 0 is written to it.
> > +
> > +	Fields have the following meanings:
> > +
> > +		version: guest has to check version before and after grabbing
> > +		time information and check that they are both equal and even.
> > +		An odd version indicates an in-progress update.
> > +
> > +		flags: At this point, always zero. May be used to indicate
> > +		changes in this structure in the future.
> > +
> > +		steal: the amount of time in which this vCPU did not run, in
> > +		nanoseconds.
> > +
> 
> The guest must initialize the entire 64-byte structure to zero before 
> enabling the feature.

I honestly don't see why. But I also don't see why not... 
Will update it.