[Patch v3 2/2] x86/microcode: Synchronize late microcode loading

[Chao Gao <chao.gao@intel.com>]

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.

This patch is also in accord with Andrew's suggestion,
"Rendezvous all online cpus in an IPI to apply the patch, and keep the
processors in until all have completed the patch.", in [1].

[1]:https://wiki.xenproject.org/wiki/XenParavirtOps/microcode_update#Run_time_microcode_updates

Signed-off-by: Chao Gao <chao.gao@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
[linux commit: a5321aec6412b20b5ad15db2d6b916c05349dbff]
[linux commit: bb8c13d61a629276a162c1d2b1a20a815cbcfbb7]
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Jun Nakajima <jun.nakajima@intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
---
v3:
 - make the 2nd parameter of wait_for_cpu() unsigned
 - correct the inverted condition in the 2nd if() in do_microcode_update().
 - when waiting for the finish of microcode update, scale the timeout by
 physical processor count other than logical thread count
 - pass the return value of stop_machine_run() to the caller.

v2:
 - Reduce the timeout from 1s to 30ms
 - update microcode with better parallelism; only one logical thread of each
 core tries to take lock and do the update.
 - remove 'error' field from struct microcode_info
 - correct coding style issues
---
 xen/arch/x86/microcode.c | 117 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 91 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/microcode.c b/xen/arch/x86/microcode.c
index 4163f50..355bc6d 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>
@@ -30,15 +31,21 @@
 #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 30000
+
 static module_t __initdata ucode_mod;
 static signed int __initdata ucode_mod_idx;
 static bool_t __initdata ucode_mod_forced;
@@ -187,9 +194,9 @@ static DEFINE_SPINLOCK(microcode_mutex);
 DEFINE_PER_CPU(struct ucode_cpu_info, ucode_cpu_info);
 
 struct microcode_info {
-    unsigned int cpu;
+    cpumask_t cpus;
+    atomic_t cpu_in, cpu_out;
     uint32_t buffer_size;
-    int error;
     char buffer[1];
 };
 
@@ -281,24 +288,56 @@ static int microcode_update_cpu(const void *buf, size_t size)
     return err;
 }
 
-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);
 
-    error = microcode_update_cpu(info->buffer, info->buffer_size);
-    if ( error )
-        info->error = error;
+    while ( atomic_read(cnt) != cpus )
+    {
+        if ( timeout <= 0 )
+        {
+            printk("Timeout when waiting for CPUs calling in\n");
+            return -EBUSY;
+        }
+        udelay(1);
+        timeout--;
+    }
 
-    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);
+    return 0;
+}
 
-    error = info->error;
-    xfree(info);
-    return error;
+static int do_microcode_update(void *_info)
+{
+    struct microcode_info *info = _info;
+    unsigned int cpu = smp_processor_id();
+    int ret;
+
+    ret = wait_for_cpus(&info->cpu_in, MICROCODE_DEFAULT_TIMEOUT);
+    if ( ret )
+        return ret;
+
+    /*
+     * Logical threads which set the first bit in cpu_sibling_mask can do
+     * the update. Other sibling threads just await the completion of
+     * microcode update.
+     */
+    if ( !cpumask_test_and_set_cpu(
+                cpumask_first(per_cpu(cpu_sibling_mask, cpu)), &info->cpus) )
+        ret = microcode_update_cpu(info->buffer, info->buffer_size);
+    /*
+     * 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(&info->cpu_out, MICROCODE_DEFAULT_TIMEOUT *
+                                       nr_phys_cpus) )
+        panic("Timeout when finishing updating microcode");
+
+    return ret;
 }
 
 int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) buf, unsigned long len)
@@ -318,26 +357,52 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) buf, unsigned long len)
 
     ret = copy_from_guest(info->buffer, buf, len);
     if ( ret != 0 )
-    {
-        xfree(info);
-        return ret;
-    }
+        goto free;
 
     info->buffer_size = len;
-    info->error = 0;
-    info->cpu = cpumask_first(&cpu_online_map);
+
+    /* 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 )
-        {
-            xfree(info);
-            return ret;
-        }
+            goto put;
     }
 
-    return continue_hypercall_on_cpu(info->cpu, do_microcode_update, info);
+    cpumask_clear(&info->cpus);
+    atomic_set(&info->cpu_in, 0);
+    atomic_set(&info->cpu_out, 0);
+
+    /*
+     * 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 feature. Right now, this is
+     *  conservative and good.
+     */
+    ret = stop_machine_run(do_microcode_update, info, NR_CPUS);
+    watchdog_enable();
+
+ put:
+    put_cpu_maps();
+ free:
+    xfree(info);
+    return ret;
 }
 
 static int __init microcode_init(void)
-- 
1.8.3.1


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