[Xen-devel] Re: [PATCH 6/7] xen: Enable PV clocksource for HVM

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2010-03-05 at 02:54 +0000, Sheng Yang wrote:
> On Friday 05 March 2010 01:40:51 Jeremy Fitzhardinge wrote:
> > On 03/04/2010 01:36 AM, Sheng Yang wrote:
> > > And enable it by default in PV extended HVM guest.
> > >
> > > Signed-off-by: Sheng Yang<sheng@xxxxxxxxxxxxxxx>
> > > ---
> > >   arch/x86/xen/hvmpv.c |   20 ++++++++++++++++++++
> > >   arch/x86/xen/time.c  |    4 +++-
> > >   2 files changed, 23 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/arch/x86/xen/hvmpv.c b/arch/x86/xen/hvmpv.c
> > > index 7a741ed..284e021 100644
> > > --- a/arch/x86/xen/hvmpv.c
> > > +++ b/arch/x86/xen/hvmpv.c
> > > @@ -43,6 +43,7 @@ static void __init xen_hvm_pv_banner(void)
> > >                   pv_info.name);
> > >           printk(KERN_INFO "Xen version: %d.%d%s\n",
> > >                   version>>  16, version&  0xffff, extra.extraversion);
> > > + printk(KERN_INFO "PV feature: PV clocksource enabled\n");
> > >   }
> > >
> > >   static int __init xen_para_available(void)
> > > @@ -117,6 +118,22 @@ static void __init init_shared_info(void)
> > >           per_cpu(xen_vcpu, 0) =&HYPERVISOR_shared_info->vcpu_info[0];
> > >   }
> > >
> > > +extern struct clocksource xen_clocksource;
> > 
> > No externs in .c files.  But aside from that, I'd prefer you export a
> > function from xen/time.c to do the clocksource registration and call it
> > from both places (even if its a one-liner).
> 
> OK
> > 
> > > +
> > > +static void __init init_pv_clocksource(void)
> > > +{
> > > + if (enable_hvm_pv(HVM_PV_CLOCK))
> > > +         BUG();
> > 
> > BUG is a bit severe.  Will it really never, ever fail?  And if it does,
> > the consequence is hardly serious; we just fall back to emulated devices.
> 
> It shouldn't fail. If we got a hypervisor without these extension, this one 
> won't be called. Because the feature should be detected by CPUID, and the 
> hypervisor would mask the unsupported features. So the guest won't see that 
> bit if it's not supported, and this one wouldn't be involved.

It is still possible that a hypervisor might want to turn on the general
extensions but not the PV clock extensions, and we can't predict what
reasons we might have for doing that in the future. A BUG() is an
awfully big hammer for a failure like this and destroys any hope of
making changes in a forward/backward compatible manner in the future.
Surely it is possible to simply continue with non-PV clock?

Ian.



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