[Xen-devel] [PATCH 4/5] xen/vcpu: Handle xen_vcpu_setup() failure in hotplug

[Ankur Arora <ankur.a.arora@xxxxxxxxxx>]

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@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 arch/x86/xen/enlighten.c     | 46 +++++++++++++++++++++++++-------------------
 arch/x86/xen/enlighten_hvm.c |  9 +++++----
 arch/x86/xen/enlighten_pv.c  | 14 +++++++++++++-
 arch/x86/xen/xen-ops.h       |  2 +-
 4 files changed, 45 insertions(+), 26 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 276cc21619ec..0e7ef69e8531 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -106,8 +106,10 @@ int xen_cpuhp_setup(int (*cpu_up_prepare_cb)(unsigned int),
        return rc >= 0 ? 0 : rc;
 }
 
-static void xen_vcpu_setup_restore(int cpu)
+static int xen_vcpu_setup_restore(int cpu)
 {
+       int rc = 0;
+
        /* Any per_cpu(xen_vcpu) is stale, so reset it */
        xen_vcpu_info_reset(cpu);
 
@@ -117,8 +119,10 @@ static void xen_vcpu_setup_restore(int cpu)
         */
        if (xen_pv_domain() ||
            (xen_hvm_domain() && cpu_online(cpu))) {
-               xen_vcpu_setup(cpu);
+               rc = xen_vcpu_setup(cpu);
        }
+
+       return rc;
 }
 
 /*
@@ -128,7 +132,7 @@ static void xen_vcpu_setup_restore(int cpu)
  */
 void xen_vcpu_restore(void)
 {
-       int cpu;
+       int cpu, rc;
 
        for_each_possible_cpu(cpu) {
                bool other_cpu = (cpu != smp_processor_id());
@@ -148,22 +152,25 @@ void xen_vcpu_restore(void)
                if (xen_pv_domain() || xen_feature(XENFEAT_hvm_safe_pvclock))
                        xen_setup_runstate_info(cpu);
 
-               xen_vcpu_setup_restore(cpu);
-
-               if (other_cpu && is_up &&
+               rc = xen_vcpu_setup_restore(cpu);
+               if (rc)
+                       pr_emerg_once("vcpu restore failed for cpu=%d err=%d. "
+                                       "System will hang.\n", cpu, rc);
+               /*
+                * In case xen_vcpu_setup_restore() fails, do not bring up the
+                * VCPU. This helps us avoid the resulting OOPS when the VCPU
+                * accesses pvclock_vcpu_time via xen_vcpu (which is NULL.)
+                * Note that this does not improve the situation much -- now the
+                * VM hangs instead of OOPSing -- with the VCPUs that did not
+                * fail, spinning in stop_machine(), waiting for the failed
+                * VCPUs to come up.
+                */
+               if (other_cpu && is_up && (rc == 0) &&
                    HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL))
                        BUG();
        }
 }
 
-static void clamp_max_cpus(void)
-{
-#ifdef CONFIG_SMP
-       if (setup_max_cpus > MAX_VIRT_CPUS)
-               setup_max_cpus = MAX_VIRT_CPUS;
-#endif
-}
-
 void xen_vcpu_info_reset(int cpu)
 {
        if (xen_vcpu_nr(cpu) < MAX_VIRT_CPUS) {
@@ -175,7 +182,7 @@ void xen_vcpu_info_reset(int cpu)
        }
 }
 
-void xen_vcpu_setup(int cpu)
+int xen_vcpu_setup(int cpu)
 {
        struct vcpu_register_vcpu_info info;
        int err;
@@ -196,7 +203,7 @@ void xen_vcpu_setup(int cpu)
         */
        if (xen_hvm_domain()) {
                if (per_cpu(xen_vcpu, cpu) == &per_cpu(xen_vcpu_info, cpu))
-                       return;
+                       return 0;
        }
 
        if (xen_have_vcpu_info_placement) {
@@ -230,11 +237,10 @@ void xen_vcpu_setup(int cpu)
                }
        }
 
-       if (!xen_have_vcpu_info_placement) {
-               if (cpu >= MAX_VIRT_CPUS)
-                       clamp_max_cpus();
+       if (!xen_have_vcpu_info_placement)
                xen_vcpu_info_reset(cpu);
-       }
+
+       return ((per_cpu(xen_vcpu, cpu) == NULL) ? -ENODEV : 0);
 }
 
 void xen_reboot(int reason)
diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
index ba1afadb2512..13b5fa1a211c 100644
--- a/arch/x86/xen/enlighten_hvm.c
+++ b/arch/x86/xen/enlighten_hvm.c
@@ -89,7 +89,7 @@ static void xen_hvm_crash_shutdown(struct pt_regs *regs)
 
 static int xen_cpu_up_prepare_hvm(unsigned int cpu)
 {
-       int rc;
+       int rc = 0;
 
        /*
         * This can happen if CPU was offlined earlier and
@@ -104,7 +104,9 @@ static int xen_cpu_up_prepare_hvm(unsigned int cpu)
                per_cpu(xen_vcpu_id, cpu) = cpu_acpi_id(cpu);
        else
                per_cpu(xen_vcpu_id, cpu) = cpu;
-       xen_vcpu_setup(cpu);
+       rc = xen_vcpu_setup(cpu);
+       if (rc)
+               return rc;
 
        if (xen_have_vector_callback && xen_feature(XENFEAT_hvm_safe_pvclock))
                xen_setup_timer(cpu);
@@ -113,9 +115,8 @@ static int xen_cpu_up_prepare_hvm(unsigned int cpu)
        if (rc) {
                WARN(1, "xen_smp_intr_init() for CPU %d failed: %d\n",
                     cpu, rc);
-               return rc;
        }
-       return 0;
+       return rc;
 }
 
 static int xen_cpu_dead_hvm(unsigned int cpu)
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 29cad193db53..e6639da11e0b 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -965,7 +965,16 @@ 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);
        }
 
        /*
@@ -1439,6 +1448,9 @@ 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);
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 90828256248b..0d5004477db6 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -78,7 +78,7 @@ bool xen_vcpu_stolen(int vcpu);
 
 extern int xen_have_vcpu_info_placement;
 
-void xen_vcpu_setup(int cpu);
+int xen_vcpu_setup(int cpu);
 void xen_vcpu_info_reset(int cpu);
 void xen_setup_vcpu_info_placement(void);
 
-- 
2.7.4


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