Scott Wood wrote:

From Power ISA 2.06B, book III-E, section 9.2.1:

Virtualized Implementation Note:

In virtualized implementations, TBU and TBL are
read-only.

But does that mean that a guest should never be allowed to modify a virtualized
timebase register, even if the hypervisor can support it?

quoted

quoted

So a generic HV mode bit is not going to help me, unless there's also a bit
that's specific to our hypervisor.  And even then, we would need some way to
differentiate among different versions of our hypervisor, in case some future
version adds timebase support. 

That's very unlikely to happen.

I know. I was just being architecturally pedantic.

Ideally we would avoid doing this sync even when not running under a
hypervisor, as long as firmware has done the sync, and kexec hasn't messed
it up.  Besides being a waste of boot time, the firmware's sync is
probably tighter since it can use a platform-specific mechanism to start all
the timebases at once.

I agree with that, but for now, I need to work around that kexec "limitation".

-- 
Timur Tabi
Linux kernel developer at Freescale