Re: Accessing host TSC from a guest kernel

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Sun, Sep 26, 2010 at 12:36:54PM +0200, Avi Kivity wrote:
>  On 09/24/2010 04:49 AM, Julien Desfossez wrote:
> >Hello,
> >
> >I'd like to access the host TSC from the inside of a guest kernel
> >and I don't really know if it's possible.
> >
> >I'm working with kvm_clock and I have been playing with the
> >pvclock_vcpu_time_info structure, but I'm not sure if I'm in the
> >right direction.
> >
> >So could you tell me if there is an efficient way to access to
> >access the host TSC (or at least the TSC_OFFSET) from a module
> >inside a guest kernel ?
> >
> >I did it with an hypercall, but it's for tracing purpose and doing
> >an hypercall every time I want to record an event, is way too
> >costly.
> 
> It isn't possible.  If you need a coherent host/guest timestamp, I
> suggest using ktime_get().

I think Avi meant that it isn't possible with stock KVM. If you hack
KVM to run the guest with 'RDTSC exiting' set to 0 and either disable
'use TSC offseting' or communicate the TSC offset to the guest out of
band, you should have efficient & consistent guest/host timestamps.

Cheers,
Muli