Re: [Xen-devel] [PATCH 1/3] xen/arm: introduce xen_read_wallclock

[Arnd Bergmann <arnd@xxxxxxxx>]

On Monday 09 November 2015 17:14:24 Stefano Stabellini wrote:
> On Mon, 9 Nov 2015, Arnd Bergmann wrote:
> > On Monday 09 November 2015 13:53:30 Stefano Stabellini wrote:
> > > On Fri, 6 Nov 2015, Arnd Bergmann wrote:
> > > > I'm not quite sure about how the split between pvclock_wall_clock and
> > > > the delta works. Normally I'd expect that pvclock_wall_clock is the 
> > > > wallclock
> > > > time as it was at boot, while delta is the number of expired nanoseconds
> > > > since boot. However it is unclear why the latter has a longer range
> > > > (539 years starting at boot, rather than 126 years starting in 1970).
> > > 
> > > Actually we already have a sec overflow field in struct shared_info on
> > > the hypervisor side, called wc_sec_hi. I just need to make use of it in
> > > Linux. See:
> > > 
> > > http://xenbits.xen.org/gitweb/?p=xen.git;a=blob_plain;f=xen/include/public/xen.h;hb=HEAD
> > > 
> > > Thanks for raising my attention to the problem,
> > 
> > Sounds good for Xen on ARM. This same interface is also used on
> > KVM, right? Does the extension also work there?
> 
> It doesn't look like it (see arch/x86/kvm/x86.c:kvm_write_wall_clock).

(adding Christoffer and Marc)

That kvm_set_msr_common() function appears to be very x86 specific though,
and I'm not sure what the ARM equivalent would be.

How does KVM get/set the system time today on ARM and ARM64? Is this
going to be done through a PSCI call based on pvclock_wall_clock in
the future?

        Arnd

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