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Re: [Xen-devel] [PATCH v5 05/10] xen/arm: Setup virtual paging for non-boot CPUs on hotplug/resume

Hi Mirela,

On 01/06/18 14:17, Mirela Simonovic wrote:
In existing code the virtual paging for non-boot CPUs is setup only on boot.
The setup is triggered from start_xen() after all CPUs are brought online.
In other words, the initialization of VTCR_EL2 register is done out of the
cpu_up/start_secondary() control flow. However, the cpu_up flow is also used
to hotplug non-boot CPUs on resume from suspend to RAM state, in which case
the virtual paging will not be configured.

With this patch the setting of paging is triggered from start_secondary()
function using cpu starting notifier (notify_cpu_starting() call). The
notifier is registered in p2m.c using init call. This has to be done with
init call rather than presmp_init because the registered callback depends
on vtcr configuration value which is setup after the presmp init calls
are executed (do_presmp_initcalls() called from start_xen()). Init calls
are executed after initial virtual paging is set up for all CPUs on boot.
This ensures that no callback can fire until the vtcr value is calculated
by Xen and virtual paging is set up initially for all CPUs. Also, this way
the virtual paging setup in boot scenario remains unchanged.

It is assumed here that after the system completed the boot, CPUs that
execute start_secondary() were booted as well when the Xen itself was
booted. According to this assumption non-boot CPUs will always be compliant
with the VTCR_EL2 value that was selected by Xen on boot.
Currently, there is no mechanism to trigger hotplugging of a CPU. This
will be added with the suspend to RAM support for ARM, where the hotplug
of non-boot CPUs will be triggered via enable_nonboot_cpus() call.

Signed-off-by: Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>

Reviewed-by: Julien Grall <julien.grall@xxxxxxx>


CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
Changes in v2:
-Fix commit message
-Save configured VTCR_EL2 value into static variable that will be used
  by non-boot CPUs on hotplug
-Add setup_virt_paging_secondary() and invoke it from start_secondary()
  if that CPU has to setup virtual paging (if the system state is not boot)

Changes in v3:
-Fix commit message
-Remove setup_virt_paging_secondary() and use notifier to setup virtual
  paging for non-boot CPU on hotplug.
-In setup_virt_paging() use vtcr static variable instead of local val
-In setup_virt_paging_one() use vtcr static variable instead of provided

Changes in v4:
-Add includes alphabetically
-Add newline before return in cpu_virt_paging_init()
-Fix indentation in cpu_virt_paging_callback() definition
-Use local val in setup_virt_paging() for calculation, assign it to vtcr
  after the calculation is done
-Remove priority initialization in the notifier structure (priority
  doesn't matter here)

Changes in v5:
-Define vtcr as uint32_t instead uint64_t
  xen/arch/arm/p2m.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++-----
  1 file changed, 48 insertions(+), 5 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index d43c3aa896..14791388ad 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -8,6 +8,8 @@
  #include <xen/iocap.h>
  #include <xen/mem_access.h>
  #include <xen/xmalloc.h>
+#include <xen/cpu.h>
+#include <xen/notifier.h>
  #include <public/vm_event.h>
  #include <asm/flushtlb.h>
  #include <asm/event.h>
@@ -1451,10 +1453,12 @@ err:
      return page;
-static void __init setup_virt_paging_one(void *data)
+/* VTCR value to be configured by all CPUs. Set only once by the boot CPU */
+static uint32_t __read_mostly vtcr;
+static void setup_virt_paging_one(void *data)
-    unsigned long val = (unsigned long)data;
-    WRITE_SYSREG32(val, VTCR_EL2);
+    WRITE_SYSREG32(vtcr, VTCR_EL2);
@@ -1538,10 +1542,49 @@ void __init setup_virt_paging(void) /* It is not allowed to concatenate a level zero root */
      BUG_ON( P2M_ROOT_LEVEL == 0 && P2M_ROOT_ORDER > 0 );
-    setup_virt_paging_one((void *)val);
-    smp_call_function(setup_virt_paging_one, (void *)val, 1);
+    vtcr = val;
+    setup_virt_paging_one(NULL);
+    smp_call_function(setup_virt_paging_one, NULL, 1);
+static int cpu_virt_paging_callback(struct notifier_block *nfb,
+                                    unsigned long action,
+                                    void *hcpu)
+    switch ( action )
+    {
+    case CPU_STARTING:
+        ASSERT(system_state != SYS_STATE_boot);
+        setup_virt_paging_one(NULL);
+        break;
+    default:
+        break;
+    }
+    return NOTIFY_DONE;
+static struct notifier_block cpu_virt_paging_nfb = {
+    .notifier_call = cpu_virt_paging_callback,
+static int __init cpu_virt_paging_init(void)
+    register_cpu_notifier(&cpu_virt_paging_nfb);
+    return 0;
+ * Initialization of the notifier has to be done at init rather than 
+ * phase because: the registered notifier is used to setup virtual paging for
+ * non-boot CPUs after the initial virtual paging for all CPUs is already 
+ * i.e. when a non-boot CPU is hotplugged after the system has booted. In other
+ * words, the notifier should be registered after the virtual paging is
+ * initially setup (setup_virt_paging() is called from start_xen()). This is
+ * required because vtcr config value has to be set before a notifier can fire.
+ */
   * Local variables:
   * mode: C

Julien Grall

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