Re: [PATCH 5/5] x86/microcode: Handle NMI's during microcode update.

["Andy Lutomirski" <luto@kernel.org>]

On Sat, Aug 13, 2022, at 3:38 PM, Ashok Raj wrote:
> Microcode updates need a guarantee that the thread sibling that is waiting
> for the update to finish on the primary core will not execute any
> instructions until the update is complete. This is required to guarantee
> any MSR or instruction that's being patched will be executed before the
> update is complete.
>
> After the stop_machine() rendezvous, an NMI handler is registered. If an
> NMI were to happen while the microcode update is not complete, the
> secondary thread will spin until the ucode update state is cleared.
>
> Couple of choices discussed are:
>
> 1. Rendezvous inside the NMI handler, and also perform the update from
>    within the handler. This seemed too risky and might cause instability
>    with the races that we would need to solve. This would be a difficult
>    choice.

I prefer choice 1.  As I understand it, Xen has done this for a while to good effect.

If I were implementing this, I would rendezvous via stop_machine as usual.  Then I would set a flag or install a handler indicating that we are doing a microcode update, send NMI-to-self, and rendezvous in the NMI handler and do the update.

> 2. Thomas (tglx) suggested that we could look into masking all the LVT
>    originating NMI's. Such as LINT1, Perf control LVT entries and such.
>    Since we are in the rendezvous loop, we don't need to worry about any
>    NMI IPI's generated by the OS.
>
>    The one we didn't have any control over is the ACPI mechanism of sending
>    notifications to kernel for Firmware First Processing (FFM). Apparently
>    it seems there is a PCH register that BIOS in SMI would write to
>    generate such an interrupt (ACPI GHES).
> 3. This is a simpler option. OS registers an NMI handler and doesn't do any
>    NMI rendezvous dance. But if an NMI were to happen, we check if any of
>    the CPUs thread siblings have an update in progress. Only those CPUs
>    would take an NMI. The thread performing the wrmsr() will only take an
>    NMI after the completion of the wrmsr 0x79 flow.
>
> Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> ---
>  arch/x86/kernel/cpu/microcode/core.c | 88 +++++++++++++++++++++++++++-
>  1 file changed, 85 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/microcode/core.c 
> b/arch/x86/kernel/cpu/microcode/core.c
> index d24e1c754c27..ec10fa2db8b1 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -40,6 +40,7 @@
>  #include <asm/cmdline.h>
>  #include <asm/setup.h>
>  #include <asm/mce.h>
> +#include <asm/nmi.h>
> 
>  #define DRIVER_VERSION	"2.2"
> 
> @@ -411,6 +412,10 @@ static int check_online_cpus(void)
> 
>  static atomic_t late_cpus_in;
>  static atomic_t late_cpus_out;
> +static atomic_t nmi_cpus;
> +static atomic_t nmi_timeouts;
> +
> +static struct cpumask cpus_in_wait;
> 
>  static int __wait_for_cpus(atomic_t *t, long long timeout)
>  {
> @@ -433,6 +438,53 @@ static int __wait_for_cpus(atomic_t *t, long long timeout)
>  	return 0;
>  }
> 
> +static int ucode_nmi_cb(unsigned int val, struct pt_regs *regs)
> +{
> +	int cpu = smp_processor_id();
> +	int timeout = 100 * NSEC_PER_USEC;
> +
> +	atomic_inc(&nmi_cpus);
> +	if (!cpumask_test_cpu(cpu, &cpus_in_wait))
> +		return NMI_DONE;
> +
> +	while (timeout < NSEC_PER_USEC) {
> +		if (timeout < NSEC_PER_USEC) {
> +			atomic_inc(&nmi_timeouts);
> +			break;
> +		}
> +		ndelay(SPINUNIT);
> +		timeout -= SPINUNIT;
> +		touch_nmi_watchdog();
> +		if (!cpumask_test_cpu(cpu, &cpus_in_wait))
> +			break;
> +	}
> +	return NMI_HANDLED;
> +}
> +
> +static void set_nmi_cpus(struct cpumask *wait_mask)
> +{
> +	int first_cpu, wait_cpu, cpu = smp_processor_id();
> +
> +	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
> +	for_each_cpu(wait_cpu, topology_sibling_cpumask(cpu)) {
> +		if (wait_cpu == first_cpu)
> +			continue;
> +		cpumask_set_cpu(wait_cpu, wait_mask);
> +	}
> +}
> +
> +static void clear_nmi_cpus(struct cpumask *wait_mask)
> +{
> +	int first_cpu, wait_cpu, cpu = smp_processor_id();
> +
> +	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
> +	for_each_cpu(wait_cpu, topology_sibling_cpumask(cpu)) {
> +		if (wait_cpu == first_cpu)
> +			continue;
> +		cpumask_clear_cpu(wait_cpu, wait_mask);
> +	}
> +}
> +
>  /*
>   * Returns:
>   * < 0 - on error
> @@ -440,7 +492,7 @@ static int __wait_for_cpus(atomic_t *t, long long timeout)
>   */
>  static int __reload_late(void *info)
>  {
> -	int cpu = smp_processor_id();
> +	int first_cpu, cpu = smp_processor_id();
>  	enum ucode_state err;
>  	int ret = 0;
> 
> @@ -459,6 +511,7 @@ static int __reload_late(void *info)
>  	 * the platform is taken to reset predictively.
>  	 */
>  	mce_set_mcip();
> +
>  	/*
>  	 * On an SMT system, it suffices to load the microcode on one sibling of
>  	 * the core because the microcode engine is shared between the threads.
> @@ -466,9 +519,17 @@ static int __reload_late(void *info)
>  	 * loading attempts happen on multiple threads of an SMT core. See
>  	 * below.
>  	 */
> +	first_cpu = cpumask_first(topology_sibling_cpumask(cpu));
> 
> -	if (cpumask_first(topology_sibling_cpumask(cpu)) == cpu)
> +	/*
> +	 * Set the CPUs that we should hold in NMI until the primary has
> +	 * completed the microcode update.
> +	 */
> +	if (first_cpu == cpu) {
> +		set_nmi_cpus(&cpus_in_wait);
>  		apply_microcode_local(&err);
> +		clear_nmi_cpus(&cpus_in_wait);
> +	}
>  	else
>  		goto wait_for_siblings;
> 
> @@ -502,20 +563,41 @@ static int __reload_late(void *info)
>   */
>  static int microcode_reload_late(void)
>  {
> -	int ret;
> +	int ret = 0;
> 
>  	pr_err("Attempting late microcode loading - it is dangerous and 
> taints the kernel.\n");
>  	pr_err("You should switch to early loading, if possible.\n");
> 
>  	atomic_set(&late_cpus_in,  0);
>  	atomic_set(&late_cpus_out, 0);
> +	atomic_set(&nmi_cpus, 0);
> +	atomic_set(&nmi_timeouts, 0);
> +	cpumask_clear(&cpus_in_wait);
> +
> +	ret = register_nmi_handler(NMI_LOCAL, ucode_nmi_cb, NMI_FLAG_FIRST,
> +				   "ucode_nmi");
> +	if (ret) {
> +		pr_err("Unable to register NMI handler\n");
> +		goto done;
> +	}
> 
>  	ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
>  	if (ret == 0)
>  		microcode_check();
> 
> +	unregister_nmi_handler(NMI_LOCAL, "ucode_nmi");
> +
> +	if (atomic_read(&nmi_cpus))
> +		pr_info("%d CPUs entered NMI while microcode update"
> +			"in progress\n", atomic_read(&nmi_cpus));
> +
> +	if (atomic_read(&nmi_timeouts))
> +		pr_err("Some CPUs [%d] entered NMI and timedout waiting for its"
> +		       " mask to be cleared\n", atomic_read(&nmi_timeouts));
> +
>  	pr_info("Reload completed, microcode revision: 0x%x\n", 
> boot_cpu_data.microcode);
> 
> +done:
>  	return ret;
>  }
> 
> -- 
> 2.32.0