xen/pvh*: Support > 32 VCPUs at domain restore

When Xen restores a PVHVM or PVH guest, its shared_info only holds
up to 32 CPUs. The hypercall VCPUOP_register_vcpu_info allows
us to setup per-page areas for VCPUs. This means we can boot
PVH* guests with more than 32 VCPUs. During restore the per-cpu
structure is allocated freshly by the hypervisor (vcpu_info_mfn is
set to INVALID_MFN) so that the newly restored guest can make a
VCPUOP_register_vcpu_info hypercall.

However, we end up triggering this condition in Xen:
/* Run this command on yourself or on other offline VCPUS. */
 if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )

which means we are unable to setup the per-cpu VCPU structures
for running VCPUS. The Linux PV code paths makes this work by
iterating over cpu_possible in xen_vcpu_restore() with:

 1) is target CPU up (VCPUOP_is_up hypercall?)
 2) if yes, then VCPUOP_down to pause it
 3) VCPUOP_register_vcpu_info
 4) if it was down, then VCPUOP_up to bring it back up

With Xen commit 192df6f9122d ("xen/x86: allow HVM guests to use
hypercalls to bring up vCPUs") this is available for non-PV guests.
As such first check if VCPUOP_is_up is actually possible before
trying this dance.

As most of this dance code is done already in xen_vcpu_restore()
let's make it callable on PV, PVH and PVHVM.

Based-on-patch-by: Konrad Wilk <konrad.wilk@oracle.com>
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, 32 insertions, 13 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 96b745e3f56c..276cc21619ec 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -106,6 +106,21 @@ int xen_cpuhp_setup(int (*cpu_up_prepare_cb)(unsigned int),
 	return rc >= 0 ? 0 : rc;
 }
 
+static void xen_vcpu_setup_restore(int cpu)
+{
+	/* Any per_cpu(xen_vcpu) is stale, so reset it */
+	xen_vcpu_info_reset(cpu);
+
+	/*
+	 * For PVH and PVHVM, setup online VCPUs only. The rest will
+	 * be handled by hotplug.
+	 */
+	if (xen_pv_domain() ||
+	    (xen_hvm_domain() && cpu_online(cpu))) {
+		xen_vcpu_setup(cpu);
+	}
+}
+
 /*
  * On restore, set the vcpu placement up again.
  * If it fails, then we're in a bad state, since
@@ -117,17 +132,23 @@ void xen_vcpu_restore(void)
 
 	for_each_possible_cpu(cpu) {
 		bool other_cpu = (cpu != smp_processor_id());
-		bool is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up, xen_vcpu_nr(cpu),
-						NULL);
+		bool is_up;
+
+		if (xen_vcpu_nr(cpu) == XEN_VCPU_ID_INVALID)
+			continue;
+
+		/* Only Xen 4.5 and higher support this. */
+		is_up = HYPERVISOR_vcpu_op(VCPUOP_is_up,
+					   xen_vcpu_nr(cpu), NULL) > 0;
 
 		if (other_cpu && is_up &&
 		    HYPERVISOR_vcpu_op(VCPUOP_down, xen_vcpu_nr(cpu), NULL))
 			BUG();
 
-		xen_setup_runstate_info(cpu);
+		if (xen_pv_domain() || xen_feature(XENFEAT_hvm_safe_pvclock))
+			xen_setup_runstate_info(cpu);
 
-		if (xen_have_vcpu_info_placement)
-			xen_vcpu_setup(cpu);
+		xen_vcpu_setup_restore(cpu);
 
 		if (other_cpu && is_up &&
 		    HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL))
@@ -163,11 +184,11 @@ void xen_vcpu_setup(int cpu)
 	BUG_ON(HYPERVISOR_shared_info == &xen_dummy_shared_info);
 
 	/*
-	 * This path is called twice on PVHVM - first during bootup via
-	 * smp_init -> xen_hvm_cpu_notify, and then if the VCPU is being
-	 * hotplugged: cpu_up -> xen_hvm_cpu_notify.
-	 * As we can only do the VCPUOP_register_vcpu_info once lets
-	 * not over-write its result.
+	 * This path is called on PVHVM at bootup (xen_hvm_smp_prepare_boot_cpu)
+	 * and at restore (xen_vcpu_restore). Also called for hotplugged
+	 * VCPUs (cpu_init -> xen_hvm_cpu_prepare_hvm).
+	 * However, the hypercall can only be done once (see below) so if a VCPU
+	 * is offlined and comes back online then let's not redo the hypercall.
 	 *
 	 * For PV it is called during restore (xen_vcpu_restore) and bootup
 	 * (xen_setup_vcpu_info_placement). The hotplug mechanism does not
@@ -178,8 +199,6 @@ void xen_vcpu_setup(int cpu)
 			return;
 	}
 
-	xen_vcpu_info_reset(cpu);
-
 	if (xen_have_vcpu_info_placement) {
 		vcpup = &per_cpu(xen_vcpu_info, cpu);
 		info.mfn = arbitrary_virt_to_mfn(vcpup);
@@ -214,7 +233,7 @@ void xen_vcpu_setup(int cpu)
 	if (!xen_have_vcpu_info_placement) {
 		if (cpu >= MAX_VIRT_CPUS)
 			clamp_max_cpus();
-		return;
+		xen_vcpu_info_reset(cpu);
 	}
 }