Re: [PATCH v2 6/6] Describe KVM_MSR_STEAL_TIME

Glauber Costa
2011-02-01T15:54:43Z

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.