[PATCHv5 15/16] ARM: hyp: initialize CNTVOFF to zero

[Mark Rutland]

On Fri, Feb 01, 2013 at 11:13:50AM +0000, Dave Martin wrote:
> On Thu, Jan 31, 2013 at 12:15:38PM +0000, Mark Rutland wrote:
> > From: Marc Zyngier <Marc.Zyngier at arm.com>
> > 
> > In order to be able to use the virtual counter in a safe way,
> > make sure it is initialized to zero before dropping to SVC.
> > 
> > Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> > Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> > Cc: Dave Martin <dave.martin at arm.com>
> > ---
> >  arch/arm/kernel/hyp-stub.S | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arm/kernel/hyp-stub.S b/arch/arm/kernel/hyp-stub.S
> > index 65b2417..455603a 100644
> > --- a/arch/arm/kernel/hyp-stub.S
> > +++ b/arch/arm/kernel/hyp-stub.S
> > @@ -152,6 +152,9 @@ THUMB(	orr	r7, #(1 << 30)	)	@ HSCTLR.TE
> >  	mrc	p15, 4, r7, c14, c1, 0	@ CNTHCTL
> >  	orr	r7, r7, #3		@ PL1PCEN | PL1PCTEN
> >  	mcr	p15, 4, r7, c14, c1, 0	@ CNTHCTL
> > +	mov	r6, #0
> > +	mov	r7, #0
> > +	mcrr	p15, 4, r6, r7, c14	@ CNTVOFF
> 
> Is this required for safety, or is it more a sanity feature?

This makes more sense with the next patch, which makes the arch_timer
driver always use the virtual counters (to avoid indirection in the fast
path and messy races with the setup of function pointers otherwise).

It's required for safety when hyp mode is enabled, and the arch_timer
driver uses the physical timers in combination with the virtual
counters. Either the driver has to apply CNTVOFF manually when setting
the physical timers, or the physical timers and virtual counters need
the same view of time (i.e. CNTVOFF == 0).

It also brings us in line with arm64, which always uses the virtual
counter for its vDSO.

> 
> The architected timer counters are supposed to be monotonic time sources
> only, so applying a random offset shouldn't really change anything.

This is true except when we want to use the physical timers as described above.

> 
> The main thing I can think of is that it is easier for the host to
> manage guests' virtual counter offsets if the host's offset is 0 (and
> we don't really want to be changing the host offset after the host kernel
> boots).

That's pretty much it. We don't want to have to further separate the handling
of the timer for host and guest. By having CNTVOFF as zero for the host, we
don't need to duplicate reading of the timers and/or incur an additional
overhead on reading them.

Thanks,
Mark.