Re: [PATCH] powerpc/85xx: disable timebase synchronization under the hypervisor

[Scott Wood <scottwood@freescale.com>]

On Wed, 22 Jun 2011 09:55:36 -0500
Timur Tabi <timur@freescale.com> wrote:

> Kumar Gala wrote:
> >> >=20
> >> > Well, not exactly. Paul wants to break that up since we're adding so=
me
> >> > primitive support for 201 HV mode too (for 970's). Last we discussed,
> >> > the plan was to go for a generic HV mode bit and a separate bit for =
the
> >> > version.
> >> >=20
> >> > Cheers,
> >> > Ben.
>=20
> > Any ETA on Paul's intro of the FTR bit?  If not I'll pull this into my =
'next' tree and we can clean up later.
>=20
> Just FYI, this particular patch is because of a limitation in the Freesca=
le
> hypervisor.  It's not because we're running in guest mode.  If the hyperv=
isor
> provided full emulation of the timebase register, then we wouldn't need t=
his
> patch.  The same can be said of KVM or any other hypervisor.

=46rom Power ISA 2.06B, book III-E, section 9.2.1:

Virtualized Implementation Note:

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

> So a generic HV mode bit is not going to help me, unless there's also a b=
it
> 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 fu=
ture
> version adds timebase support.=20

That's very unlikely to happen.

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.

-Scott