Re: [Xen-devel] [PATCH v10 14/16] microcode: rendezvous CPUs in NMI handler and load ucode

[Jan Beulich <jbeulich@suse.com>]

On 12.09.2019 09:22, Chao Gao wrote:
> When one core is loading ucode, handling NMI on sibling threads or
> on other cores in the system might be problematic. By rendezvousing
> all CPUs in NMI handler, it prevents NMI acceptance during ucode
> loading.
> 
> Basically, some work previously done in stop_machine context is
> moved to NMI handler. Primary threads call in and load ucode in
> NMI handler. Secondary threads wait for the completion of ucode
> loading on all CPU cores. An option is introduced to disable this
> behavior.
> 
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>



> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -2056,6 +2056,16 @@ microcode in the cpio name space must be:
>    - on Intel: kernel/x86/microcode/GenuineIntel.bin
>    - on AMD  : kernel/x86/microcode/AuthenticAMD.bin
>  
> +### ucode_loading_in_nmi (x86)
> +> `= <boolean>`
> +
> +> Default: `true`
> +
> +When one CPU is loading ucode, handling NMIs on sibling threads or threads on
> +other cores might cause problems. By default, all CPUs rendezvous in NMI handler
> +and load ucode. This option provides a way to disable it in case of some CPUs
> +don't allow ucode loading in NMI handler.

We already have "ucode=", why don't you extend it to allow "ucode=nmi"
and "ucode=no-nmi"? (In any event, please no underscores in new
command line options - use hyphens if necessary.)

> @@ -232,6 +237,7 @@ DEFINE_PER_CPU(struct cpu_signature, cpu_sig);
>   */
>  static cpumask_t cpu_callin_map;
>  static atomic_t cpu_out, cpu_updated;
> +const struct microcode_patch *nmi_patch;

static

> @@ -354,6 +360,50 @@ static void set_state(unsigned int state)
>      smp_wmb();
>  }
>  
> +static int secondary_thread_work(void)
> +{
> +    cpumask_set_cpu(smp_processor_id(), &cpu_callin_map);
> +
> +    return wait_for_state(LOADING_EXIT) ? 0 : -EBUSY;
> +}
> +
> +static int primary_thread_work(const struct microcode_patch *patch)

I think it would be nice if both functions carried "nmi" in their
names - how about {primary,secondary}_nmi_work()? Or wait - the
primary one gets used outside of NMI as well, so I'm fine with its
name. The secondary one, otoh, is NMI-specific and also its only
caller doesn't care about the return value, so I'd suggest making
it return void alongside adding some form of "nmi" to its name. Or,
perhaps even better, have secondary_thread_fn() call it, moving the
cpu_sig update here (and of course then there shouldn't be any
"nmi" added to its name).

> +static int microcode_nmi_callback(const struct cpu_user_regs *regs, int cpu)
> +{
> +    unsigned int primary = cpumask_first(this_cpu(cpu_sibling_mask));
> +    unsigned int controller = cpumask_first(&cpu_online_map);
> +
> +    /* System-generated NMI, will be ignored */
> +    if ( loading_state != LOADING_CALLIN )
> +        return 0;

I'm not happy at all to see NMIs being ignored. But by returning
zero, you do _not_ ignore it. Did you perhaps mean "will be ignored
here", in which case perhaps better "leave to main handler"? And
for the comment to extend to the other two conditions right below,
I think it would be better to combine them all into a single if().

Also, throughout the series, I think you want to consistently use
ACCESS_ONCE() for reads/writes from/to loading_state.

> +    if ( cpu == controller || (!opt_ucode_loading_in_nmi && cpu == primary) )
> +        return 0;

Why not

    if ( cpu == controller || !opt_ucode_loading_in_nmi )
        return 0;

? (And then, there being just a single use each in this function, I
don't think there's a need for the two local variables.)

> @@ -361,10 +411,7 @@ static int secondary_thread_fn(void)
>      if ( !wait_for_state(LOADING_CALLIN) )
>          return -EBUSY;
>  
> -    cpumask_set_cpu(smp_processor_id(), &cpu_callin_map);
> -
> -    if ( !wait_for_state(LOADING_EXIT) )
> -        return -EBUSY;
> +    self_nmi();

Loosing the -EBUSY indication here isn't very nice. Perhaps this
should be conveyed via a per-CPU variable?

> @@ -379,15 +426,10 @@ static int primary_thread_fn(const struct microcode_patch *patch)
>      if ( !wait_for_state(LOADING_CALLIN) )
>          return -EBUSY;
>  
> -    cpumask_set_cpu(smp_processor_id(), &cpu_callin_map);
> -
> -    if ( !wait_for_state(LOADING_ENTER) )
> -        return -EBUSY;
> -
> -    ret = microcode_ops->apply_microcode(patch);
> -    if ( !ret )
> -        atomic_inc(&cpu_updated);
> -    atomic_inc(&cpu_out);
> +    if ( opt_ucode_loading_in_nmi )
> +        self_nmi();

Same here.

> @@ -404,6 +447,9 @@ static int control_thread_fn(const struct microcode_patch *patch)
>       */
>      watchdog_disable();
>  
> +    nmi_patch = patch;
> +    saved_nmi_callback = set_nmi_callback(microcode_nmi_callback);

Shouldn't there be smb_wmb() between these two?

> @@ -458,6 +513,7 @@ static int control_thread_fn(const struct microcode_patch *patch)
>      /* Mark loading is done to unblock other threads */
>      set_state(LOADING_EXIT);
>  
> +    set_nmi_callback(saved_nmi_callback);

To be on the safe side, I think you also want to clear nmi_patch again.
Or maybe even better not clear it, but set it to a non-NULL value which,
when accessed, would trap (e.g. ZERO_BLOCK_PTR). This value should then
also be the variable's initializer.

> @@ -522,6 +578,13 @@ int microcode_update(XEN_GUEST_HANDLE_PARAM(const_void) buf, unsigned long len)
>          goto free;
>      }
>  
> +    /*
> +     * CPUs except the first online CPU would send a fake (self) NMI to
> +     * rendezvous in NMI handler. But a fake NMI to nmi_cpu may trigger
> +     * unknown_nmi_error(). It ensures nmi_cpu won't receive a fake NMI.
> +     */
> +    ASSERT( !cpu_online(nmi_cpu) || nmi_cpu == cpumask_first(&cpu_online_map) );

Please drop the blanks immediately inside the parentheses.

As to the left side of the || - is this really needed? It surely would
be very wrong (but entirely unrelated to ucode loading) if the CPU to
receive platform NMIs was offline.

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -126,6 +126,8 @@ boolean_param("ler", opt_ler);
>  /* LastExceptionFromIP on this hardware.  Zero if LER is not in use. */
>  unsigned int __read_mostly ler_msr;
>  
> +unsigned int __read_mostly nmi_cpu;

Since this variable (for now) is never written to it should gain a
comment saying why this is, and perhaps it would then also better be
const rather than __read_mostly.

Jan

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