Only the IPI-related functions in the smp_ops should be conditional
on the vector callback being available. The rest should still happen:

 • xen_hvm_smp_prepare_boot_cpu()

   This function does two things, both of which should still happen if
   there is no vector callback support.

   The call to xen_vcpu_setup() for vCPU0 should still happen as it just
   sets up the vcpu_info for CPU0. That does happen for the secondary
   vCPUs too, from xen_cpu_up_prepare_hvm().

   The second thing it does is call xen_init_spinlocks(), which perhaps
   counter-intuitively should *also* still be happening in the case
   without vector callbacks, so that it can clear its local xen_pvspin
   flag and disable the virt_spin_lock_key accordingly.

   Checking xen_have_vector_callback in xen_init_spinlocks() itself
   would affect PV guests, so set the global nopvspin flag in
   xen_hvm_smp_init() instead, when vector callbacks aren't available.

 • xen_hvm_smp_prepare_cpus()

   This does some IPI-related setup by calling xen_smp_intr_init() and
   xen_init_lock_cpu(), which can be made conditional. And it sets the
   xen_vcpu_id to XEN_VCPU_ID_INVALID for all possible CPUS, which does
   need to happen.

 • xen_smp_cpus_done()

   This offlines any vCPUs which doesn't fit in the global shared_info
   page, if separate vcpu_info placement isn't available. That part also
   needs to happen regardless of vector callback support.

 • xen_hvm_cpu_die()

   This doesn't actually do anything other than commin_cpu_die() right
   right now in the !vector_callback case; all three teardown functions
   it calls should be no-ops. But to guard against future regressions
   it's useful to call it anyway, and for it to explicitly check for
   xen_have_vector_callback before calling those additional functions.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210106153958.584169-6-dwmw2@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>