Re: [Xen-devel] [PATCH] xen: schedule: allow dom0 vCPUs to be re-pinned when dom0_vcpus_pin is set

[Matt Wilson <msw@xxxxxxxxxx>]

On Wed, Dec 05, 2012 at 04:25:36PM +0000, Jan Beulich wrote:
> >>> On 05.12.12 at 16:59, Matt Wilson <msw@xxxxxxxxxx> wrote:
> > On Wed, Dec 05, 2012 at 10:44:04AM +0000, Andrew Cooper wrote:
> >> On 05/12/12 06:02, Matt Wilson wrote:
> >> > An administrator may want Xen to pin dom0 vCPUs to pCPUs 1:1 at boot,
> >> > but still have the flexibility to change the configuration later.
> >> > There's no logic that keys off of domain->is_pinned outside of
> >> > sched_init_vcpu() and vcpu_set_affinity(). By adjusting the
> >> > is_pinned_vcpu() macro to only check for a single CPU set in the
> >> > cpu_affinity mask, dom0 vCPUs can safely be re-pinned after the system
> >> > boots.
> >> 
> >> Sadly this patch will break things.  There are certain callers of
> >> is_pinned_vcpu() which rely on the value to allow access to certain
> >> power related MSRs, which is where the requirement for never permitting
> >> an update of the affinity mask comes from.
> > 
> > If this is true, the existing is_pinned_vcpu() test is broken:
> > 
> >    #define is_pinned_vcpu(v) ((v)->domain->is_pinned || \
> >                               cpumask_weight((v)->cpu_affinity) == 1)
> > 
> > It's && not ||. So if someone pins dom0 vCPUs to pCPUs 1:1 after boot,
> > the MSR traps will suddenly start working.
> > 
> > See commit: 
> > http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=cc0854dd 
> 
> I don't see what's wrong here. Certain things merely require the
> pCPU that a vCPU runs on to be stable, which is what the test
> above is for.

Me either. That said, are you willing to Ack and commit my patch that
started this thread?

Thanks,

Matt

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