Re: [PATCH v2 1/3] x86/cpufeatures: Enumerate user wait instructions

From: Fenghua Yu
Date: Tue Feb 26 2019 - 15:48:45 EST


On Wed, Feb 20, 2019 at 10:37:27PM -0800, Andy Lutomirski wrote:
> On Wed, Feb 20, 2019 at 7:44 PM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> >
> > From: Fenghua Yu <fenghua.yu@xxxxxxxxx>
>
> >
> > From patchwork Wed Jan 16 21:18:41 2019
> > Content-Type: text/plain; charset="utf-8"
>
> [snipped more stuff like this]
>
> What happened here?
>
> > +/* Return value that will be used to set umwait control MSR */
> > +static inline u32 umwait_control_val(void)
> > +{
> > + /*
> > + * Enable or disable C0.2 (bit 0) based on global setting on all CPUs.
> > + * When bit 0 is 1, C0.2 is disabled. Otherwise, C0.2 is enabled.
> > + * So value in bit 0 is opposite of umwait_enable_c0_2.
> > + */
> > + return ~umwait_enable_c0_2 & UMWAIT_CONTROL_C02_MASK;
> > +}
>
> This function is horribly named. How about something like
> umwait_compute_msr_value() or something liek that? Also, what
> happened to the maximum wait time?
>
> > +
> > +static ssize_t umwait_enable_c0_2_show(struct device *dev,
> > + struct device_attribute *attr,
> > + char *buf)
> > +{
> > + return sprintf(buf, "%d\n", umwait_enable_c0_2);
>
> I realize that it's traditional to totally ignore races in sysfs and
> such, but it's a bad tradition. Please either READ_ONCE it with a
> comment or take the mutex.
>
> > +static ssize_t umwait_enable_c0_2_store(struct device *dev,
> > + struct device_attribute *attr,
> > + const char *buf, size_t count)
> > +{
> > + int enable_c0_2, cpu, ret;
> > + u32 msr_val;
> > +
> > + ret = kstrtou32(buf, 10, &enable_c0_2);
> > + if (ret)
> > + return ret;
> > +
> > + if (enable_c0_2 != 1 && enable_c0_2 != 0)
> > + return -EINVAL;
>
> How about if (enable_c0_2 > 1)?
>
> > +
> > + mutex_lock(&umwait_lock);
> > +
> > + umwait_enable_c0_2 = enable_c0_2;
> > + msr_val = umwait_control_val();
> > + get_online_cpus();
> > + /* All CPUs have same umwait control setting */
> > + for_each_online_cpu(cpu)
> > + wrmsr_on_cpu(cpu, MSR_IA32_UMWAIT_CONTROL, msr_val, 0);
> > + put_online_cpus();
> > +
> > + mutex_unlock(&umwait_lock);
>
> Please factor this thing out into a helper like
> umwait_update_all_cpus(). That helper can assert that the lock is
> held.
>
> > +/* Set up umwait control MSR on this CPU using the current global setting. */
> > +static int umwait_cpu_online(unsigned int cpu)
> > +{
> > + u32 msr_val;
> > +
> > + mutex_lock(&umwait_lock);
> > +
> > + msr_val = umwait_control_val();
> > + wrmsr(MSR_IA32_UMWAIT_CONTROL, msr_val, 0);
> > +
> > + mutex_unlock(&umwait_lock);
> > +
> > + return 0;
> > +}
> > +
> > +static int __init umwait_init(void)
> > +{
> > + struct device *dev;
> > + int ret;
> > +
> > + if (!boot_cpu_has(X86_FEATURE_WAITPKG))
> > + return -ENODEV;
> > +
> > + /* Add CPU global user wait interface to control umwait. */
> > + dev = cpu_subsys.dev_root;
> > + ret = sysfs_create_group(&dev->kobj, &umwait_attr_group);
> > + if (ret)
> > + return ret;
> > +
> > + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "umwait/intel:online",
> > + umwait_cpu_online, NULL);
>
> This hotplug notifier thing is awful. Thomas, do we have a function
> that gets called every time a CPU is brought up (via BSP boot, AP
> boot, hotplug, hibernation resume, etc) where we can just put all
> these things? cpu_init() is almost appropriate, except that it's
> called at somewhat erratic times (quite different for BSP and AP IIRC)
> and it's not called AFAICT during hibernation restore. I suppose we
> could add a new thing that is called by cpu_init() and
> restore_processor_state().
>
> Also, surely you should actually write the MSR in this function, too.

Seems the current patch set misses pm_notifier for hibernation on BSP.
All APs are all updated by the online funciton in the current patch set.
If adding hiberation pm_notifier to update MSR 0xe1 on BSP, is that good
enough?

Thanks.

-Fenghua