[Xen-devel] [PATCH v5 7/8] x86/microcode: Synchronize late microcode loading

[Chao Gao <chao.gao@xxxxxxxxx>]

This patch ports microcode improvement patches from linux kernel.

Before you read any further: the early loading method is still the
preferred one and you should always do that. The following patch is
improving the late loading mechanism for long running jobs and cloud use
cases.

Gather all cores and serialize the microcode update on them by doing it
one-by-one to make the late update process as reliable as possible and
avoid potential issues caused by the microcode update.

Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
Tested-by: Chao Gao <chao.gao@xxxxxxxxx>
[linux commit: a5321aec6412b20b5ad15db2d6b916c05349dbff]
[linux commit: bb8c13d61a629276a162c1d2b1a20a815cbcfbb7]
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/microcode.c | 125 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 98 insertions(+), 27 deletions(-)

diff --git a/xen/arch/x86/microcode.c b/xen/arch/x86/microcode.c
index 3c2274f..b7b20cf 100644
--- a/xen/arch/x86/microcode.c
+++ b/xen/arch/x86/microcode.c
@@ -22,6 +22,7 @@
  */
 
 #include <xen/cpu.h>
+#include <xen/cpumask.h>
 #include <xen/lib.h>
 #include <xen/kernel.h>
 #include <xen/init.h>
@@ -30,18 +31,25 @@
 #include <xen/smp.h>
 #include <xen/softirq.h>
 #include <xen/spinlock.h>
+#include <xen/stop_machine.h>
 #include <xen/tasklet.h>
 #include <xen/guest_access.h>
 #include <xen/earlycpio.h>
+#include <xen/watchdog.h>
 
+#include <asm/delay.h>
 #include <asm/msr.h>
 #include <asm/processor.h>
 #include <asm/setup.h>
 #include <asm/microcode.h>
 
+/* By default, wait for 30000us */
+#define MICROCODE_DEFAULT_TIMEOUT_US 30000
+
 static module_t __initdata ucode_mod;
 static signed int __initdata ucode_mod_idx;
 static bool_t __initdata ucode_mod_forced;
+static unsigned int nr_cores;
 
 /*
  * If we scan the initramfs.cpio for the early microcode code
@@ -188,10 +196,11 @@ static DEFINE_SPINLOCK(microcode_mutex);
 
 DEFINE_PER_CPU(struct ucode_cpu_info, ucode_cpu_info);
 
-struct microcode_info {
-    unsigned int cpu;
-    int error;
-};
+/*
+ * Count the CPUs that have entered and exited the rendezvous
+ * during late microcode update.
+ */
+static atomic_t cpu_in, cpu_out;
 
 static void microcode_fini_cpu(unsigned int cpu)
 {
@@ -290,30 +299,60 @@ int microcode_resume_cpu(unsigned int cpu)
     return microcode_ops ? microcode_update_cpu() : 0;
 }
 
-static long do_microcode_update(void *_info)
+/* Wait for all CPUs to rendezvous with a timeout (us) */
+static int wait_for_cpus(atomic_t *cnt, unsigned int timeout)
 {
-    struct microcode_info *info = _info;
-    int error;
+    unsigned int cpus = num_online_cpus();
 
-    BUG_ON(info->cpu != smp_processor_id());
+    atomic_inc(cnt);
+
+    while ( atomic_read(cnt) != cpus )
+    {
+        if ( timeout <= 0 )
+        {
+            printk("CPU%d: Timeout when waiting for CPUs calling in\n",
+                   smp_processor_id());
+            return -EBUSY;
+        }
+        udelay(1);
+        timeout--;
+    }
+
+    return 0;
+}
+
+static int do_microcode_update(void *unused)
+{
+    unsigned int cpu = smp_processor_id();
+    int ret;
 
-    error = microcode_update_cpu();
-    if ( error )
-        info->error = error;
+    ret = wait_for_cpus(&cpu_in, MICROCODE_DEFAULT_TIMEOUT_US);
+    if ( ret )
+        return ret;
 
-    info->cpu = cpumask_next(info->cpu, &cpu_online_map);
-    if ( info->cpu < nr_cpu_ids )
-        return continue_hypercall_on_cpu(info->cpu, do_microcode_update, info);
+    /*
+     * Initiate an update on all processors which don't have an online sibling
+     * thread with a lower thread id. Other sibling threads just await the
+     * completion of microcode update.
+     */
+    if ( cpu == cpumask_first(per_cpu(cpu_sibling_mask, cpu)) )
+        ret = microcode_update_cpu();
+    /*
+     * Increase the wait timeout to a safe value here since we're serializing
+     * the microcode update and that could take a while on a large number of
+     * CPUs. And that is fine as the *actual* timeout will be determined by
+     * the last CPU finished updating and thus cut short
+     */
+    if ( wait_for_cpus(&cpu_out, MICROCODE_DEFAULT_TIMEOUT_US * nr_cores) )
+        panic("Timeout when finishing updating microcode");
 
-    error = info->error;
-    xfree(info);
-    return error;
+    return ret;
 }
 
 int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) buf, unsigned long len)
 {
     int ret;
-    struct microcode_info *info;
+    unsigned int cpu;
     void * buffer;
 
     if ( len != (uint32_t)len )
@@ -322,9 +361,8 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) 
buf, unsigned long len)
     if ( microcode_ops == NULL )
         return -EINVAL;
 
-    info = xmalloc(struct microcode_info);
     buffer = xmalloc_bytes(len);
-    if ( !info || !buffer )
+    if ( !buffer )
     {
         ret = -ENOMEM;
         goto free;
@@ -334,28 +372,61 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) 
buf, unsigned long len)
     if ( ret != 0 )
         goto free;
 
+    /* cpu_online_map must not change during update */
+    if ( !get_cpu_maps() )
+    {
+        ret = -EBUSY;
+        goto free;
+    }
+
     if ( microcode_ops->start_update )
     {
         ret = microcode_ops->start_update();
         if ( ret != 0 )
-            goto free;
+            goto put;
     }
 
     ret = parse_microcode_blob(buffer, len);
     if ( ret <= 0 )
     {
         printk(XENLOG_ERR "No valid or newer ucode found. Update abort!\n");
-        xfree(info);
-        return -EINVAL;
+        ret = -EINVAL;
+        goto put;
     }
 
-    info->error = 0;
-    info->cpu = cpumask_first(&cpu_online_map);
+    atomic_set(&cpu_in, 0);
+    atomic_set(&cpu_out, 0);
+
+    /* Calculate the number of online CPU core */
+    nr_cores = 0;
+    for_each_online_cpu(cpu)
+        if ( cpu == cpumask_first(per_cpu(cpu_sibling_mask, cpu)) )
+            nr_cores++;
+
+    printk(XENLOG_INFO "%d cores are to update their microcode\n", nr_cores);
 
-    return continue_hypercall_on_cpu(info->cpu, do_microcode_update, info);
+    /*
+     * We intend to disable interrupt for long time, which may lead to
+     * watchdog timeout.
+     */
+    watchdog_disable();
+    /*
+     * Late loading dance. Why the heavy-handed stop_machine effort?
+     *
+     * - HT siblings must be idle and not execute other code while the other
+     *   sibling is loading microcode in order to avoid any negative
+     *   interactions cause by the loading.
+     *
+     * - In addition, microcode update on the cores must be serialized until
+     *   this requirement can be relaxed in the future. Right now, this is
+     *   conservative and good.
+     */
+    ret = stop_machine_run(do_microcode_update, NULL, NR_CPUS);
+    watchdog_enable();
 
+ put:
+    put_cpu_maps();
  free:
-    xfree(info);
     xfree(buffer);
     return ret;
 }
-- 
1.8.3.1


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