Accessing host TSC from a guest kernel

Accessing host TSC from a guest kernel

[Julien Desfossez]

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.

Thanks,

Julien

Re: Accessing host TSC from a guest kernel

[Avi Kivity]

  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().

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

Re: Accessing host TSC from a guest kernel

[Muli Ben-Yehuda]

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