Re: [Xen-devel] [PATCH v7 08/10] x86/microcode: Synchronize late microcode loading

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, May 27, 2019 at 04:31:29PM +0800, Chao Gao wrote:
> 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>
> ---
> Changes in v7:
>  - Check whether 'timeout' is 0 rather than "<=0" since it is unsigned int.
>  - reword the comment above microcode_update_cpu() to clearly state that
>  one thread per core should do the update.
> 
> Changes in v6:
>  - Use one timeout period for rendezvous stage and another for update stage.
>  - scale time to wait by the number of remaining cpus to respond.
>    It helps to find something wrong earlier and thus we can reboot the
>    system earlier.
> ---
>  xen/arch/x86/microcode.c | 171 
> ++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 155 insertions(+), 16 deletions(-)
> 
> diff --git a/xen/arch/x86/microcode.c b/xen/arch/x86/microcode.c
> index 23cf550..f4a417e 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 +31,34 @@
>  #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>
>  
> +/*
> + * Before performing a late microcode update on any thread, we
> + * rendezvous all cpus in stop_machine context. The timeout for
> + * waiting for cpu rendezvous is 30ms. It is the timeout used by
> + * live patching
> + */
> +#define MICROCODE_CALLIN_TIMEOUT_US 30000
> +
> +/*
> + * Timeout for each thread to complete update is set to 1s. It is a
> + * conservative choice considering all possible interference (for
> + * instance, sometimes wbinvd takes relative long time). And a perfect
> + * timeout doesn't help a lot except an early shutdown.

I would remove the "And a perfect..." sentence. I don't think it makes
much sense to speak about "perfect timeouts".

> + */
> +#define MICROCODE_UPDATE_TIMEOUT_US 1000000
> +
>  static module_t __initdata ucode_mod;
>  static signed int __initdata ucode_mod_idx;
>  static bool_t __initdata ucode_mod_forced;
> @@ -190,6 +210,12 @@ static DEFINE_SPINLOCK(microcode_mutex);
>  DEFINE_PER_CPU(struct cpu_signature, cpu_sig);
>  
>  /*
> + * Count the CPUs that have entered, exited the rendezvous and succeeded in
> + * microcode update during late microcode update respectively.
> + */
> +static atomic_t cpu_in, cpu_out, cpu_updated;
> +
> +/*
>   * Return the patch with the highest revision id among all matching
>   * patches in the blob. Return NULL if no suitable patch.
>   */
> @@ -270,31 +296,90 @@ bool microcode_update_cache(struct microcode_patch 
> *patch)
>      return true;
>  }
>  
> -static long do_microcode_update(void *patch)
> +/* Wait for CPUs to rendezvous with a timeout (us) */
> +static int wait_for_cpus(atomic_t *cnt, unsigned int expect,
> +                         unsigned int timeout)
>  {
> -    int error, cpu;
> -
> -    error = microcode_update_cpu(patch);
> -    if ( error )
> +    while ( atomic_read(cnt) < expect )
>      {
> -        microcode_ops->free_patch(microcode_cache);
> -        return error;
> +        if ( !timeout )
> +        {
> +            printk("CPU%d: Timeout when waiting for CPUs calling in\n",
> +                   smp_processor_id());
> +            return -EBUSY;
> +        }
> +        udelay(1);
> +        timeout--;

Nit: you could do the decrement inside the if condition.

>      }
>  
> +    return 0;
> +}
>  
> -    cpu = cpumask_next(smp_processor_id(), &cpu_online_map);
> -    if ( cpu < nr_cpu_ids )
> -        return continue_hypercall_on_cpu(cpu, do_microcode_update, patch);
> +static int do_microcode_update(void *patch)
> +{
> +    unsigned int cpu = smp_processor_id();
> +    unsigned int cpu_nr = num_online_cpus();
> +    unsigned int finished;
> +    int ret;
> +    static bool error;
>  
> -    microcode_update_cache(patch);
> +    atomic_inc(&cpu_in);
> +    ret = wait_for_cpus(&cpu_in, cpu_nr, MICROCODE_CALLIN_TIMEOUT_US);
> +    if ( ret )
> +        return ret;
>  
> -    return error;
> +    ret = microcode_ops->collect_cpu_info(&this_cpu(cpu_sig));
> +    /*
> +     * Load microcode update on only one logical processor per core.
> +     * Here, among logical processors of a core, the one with the
> +     * lowest thread id is chosen to perform the loading.
> +     */
> +    if ( !ret && (cpu == cpumask_first(per_cpu(cpu_sibling_mask, cpu))) )
> +    {
> +        ret = microcode_ops->apply_microcode(patch);
> +        if ( !ret )
> +            atomic_inc(&cpu_updated);
> +    }
> +    /*
> +     * 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

It's likely me missing something, but where is this serialization
being done?

I assume it's done by apply_microcode because do_microcode_update
doesn't do any serialization of microcode loading.

> +     */
> +    atomic_inc(&cpu_out);
> +    finished = atomic_read(&cpu_out);
> +    while ( !error && finished != cpu_nr )
> +    {
> +        /*
> +         * During each timeout interval, at least a CPU is expected to
> +         * finish its update. Otherwise, something goes wrong.
> +         */
> +        if ( wait_for_cpus(&cpu_out, finished + 1,
> +                           MICROCODE_UPDATE_TIMEOUT_US) && !error )
> +        {
> +            error = true;

I'm not sure I see the point of the error variable, you already bring
the system down with panic. If the intention is to prevent multiple
panics from different threads then you need to use some kind of
atomic fetch and set or else the code is racy.

> +            panic("Timeout when finishing updating microcode (finished 
> %d/%d)",

Both finished and cpu_nr are unsigned ints, hence you should use %u
instead of %d.

> +                  finished, cpu_nr);
> +        }

This whole loop seems to be designed for serialized microcode
application, which is not the case with the current implementation
where microcode is updated in parallel on all the cores?

IMO you should just wait for MICROCODE_UPDATE_TIMEOUT_US a single
time.

> +        finished = atomic_read(&cpu_out);
> +    }
> +
> +    /*
> +     * Refresh CPU signature (revision) on threads which didn't call
> +     * apply_microcode().
> +     */
> +    if ( cpu != cpumask_first(per_cpu(cpu_sibling_mask, cpu)) )
> +        ret = microcode_ops->collect_cpu_info(&this_cpu(cpu_sig));
> +
> +    return ret;
>  }
>  
>  int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) buf, unsigned long 
> len)
>  {
>      int ret;
>      void *buffer;
> +    unsigned int cpu, nr_cores;
>      struct microcode_patch *patch;
>  
>      if ( len != (uint32_t)len )
> @@ -316,11 +401,18 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) 
> buf, unsigned long len)
>          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;
>      }
>  
>      patch = microcode_parse_blob(buffer, len);
> @@ -337,12 +429,59 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) 
> buf, unsigned long len)
>          if ( patch )
>              microcode_ops->free_patch(patch);
>          ret = -EINVAL;
> -        goto free;
> +        goto put;
>      }
>  
> -    ret = continue_hypercall_on_cpu(cpumask_first(&cpu_online_map),
> -                                    do_microcode_update, patch);
> +    atomic_set(&cpu_in, 0);
> +    atomic_set(&cpu_out, 0);
> +    atomic_set(&cpu_updated, 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);

Same here, nr_cores is unsigned.

> +
> +    /*
> +     * 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, patch, NR_CPUS);
> +    watchdog_enable();
> +
> +    if ( atomic_read(&cpu_updated) == nr_cores )
> +    {
> +        spin_lock(&microcode_mutex);
> +        microcode_update_cache(patch);
> +        spin_unlock(&microcode_mutex);
> +    }
> +    else if ( atomic_read(&cpu_updated) == 0 )
> +        microcode_ops->free_patch(patch);
> +    else
> +    {
> +        printk("Updating microcode succeeded on part of CPUs and failed on\n"

I would prefix this with XENLOG_ERR and an explicit "ERROR: " prefix
in the format string.

Thanks, Roger.

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