xen/vcpu: Handle xen_vcpu_setup() failure in hotplug

The hypercall VCPUOP_register_vcpu_info can fail. This failure is
handled by making per_cpu(xen_vcpu, cpu) point to its shared_info
slot and those without one (cpu >= MAX_VIRT_CPUS) be NULL.

For PVH/PVHVM, this is not enough, because we also need to pull
these VCPUs out of circulation.

Fix for PVH/PVHVM: on registration failure in the cpuhp prepare
callback (xen_cpu_up_prepare_hvm()), return an error to the cpuhp
state-machine so it can fail the CPU init.

Fix for PV: the registration happens before smp_init(), so, in the
failure case we clamp setup_max_cpus and limit the number of VCPUs
that smp_init() will bring-up to MAX_VIRT_CPUS.
This is functionally correct but it makes the code a bit simpler
if we get rid of this explicit clamping: for VCPUs that don't have
valid xen_vcpu, fail the CPU init in the cpuhp prepare callback
(xen_cpu_up_prepare_pv()).

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 9a0714dbddfa..bc44d2175459 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -958,7 +958,16 @@ void __ref xen_setup_vcpu_info_placement(void)
 	for_each_possible_cpu(cpu) {
 		/* Set up direct vCPU id mapping for PV guests. */
 		per_cpu(xen_vcpu_id, cpu) = cpu;
-		xen_vcpu_setup(cpu);
+
+		/*
+		 * xen_vcpu_setup(cpu) can fail  -- in which case it
+		 * falls back to the shared_info version for cpus
+		 * where xen_vcpu_nr(cpu) < MAX_VIRT_CPUS.
+		 *
+		 * xen_cpu_up_prepare_pv() handles the rest by failing
+		 * them in hotplug.
+		 */
+		(void) xen_vcpu_setup(cpu);
 	}
 
 	/*
@@ -1432,6 +1441,9 @@ static int xen_cpu_up_prepare_pv(unsigned int cpu)
 {
 	int rc;
 
+	if (per_cpu(xen_vcpu, cpu) == NULL)
+		return -ENODEV;
+
 	xen_setup_timer(cpu);
 
 	rc = xen_smp_intr_init(cpu);