Date:   Sun, 9 Jun 2019 23:02:34 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait
 C0.2 state

On Sun, Jun 09, 2019 at 09:24:18PM -0700, Andy Lutomirski wrote:
> On Sun, Jun 9, 2019 at 9:02 PM Fenghua Yu <fenghua.yu@...el.com> wrote:
> >
> > On Sat, Jun 08, 2019 at 03:50:32PM -0700, Andy Lutomirski wrote:
> > > On Fri, Jun 7, 2019 at 3:10 PM Fenghua Yu <fenghua.yu@...el.com> wrote:
> > > >
> > > > C0.2 state in umwait and tpause instructions can be enabled or disabled
> > > > on a processor through IA32_UMWAIT_CONTROL MSR register.
> > > >
> > > > By default, C0.2 is enabled and the user wait instructions result in
> > > > lower power consumption with slower wakeup time.
> > > >
> > > > But in real time systems which require faster wakeup time although power
> > > > savings could be smaller, the administrator needs to disable C0.2 and all
> > > > C0.2 requests from user applications revert to C0.1.
> > > >
> > > > A sysfs interface "/sys/devices/system/cpu/umwait_control/enable_c02" is
> > > > created to allow the administrator to control C0.2 state during run time.
> > >
> > > This looks better than the previous version.  I think the locking is
> > > still rather confused.  You have a mutex that you hold while changing
> > > the value, which is entirely reasonable.  But, of the code paths that
> > > write the MSR, only one takes the mutex.
> > >
> > > I think you should consider making a function that just does:
> > >
> > > wrmsr(MSR_IA32_UMWAIT_CONTROL, READ_ONCE(umwait_control_cached), 0);
> > >
> > > and using it in all the places that update the MSR.  The only thing
> > > that should need the lock is the sysfs code to avoid accidentally
> > > corrupting the value, but that code should also use WRITE_ONCE to do
> > > its update.
> >
> > Based on the comment, the illustrative CPU online and enable_c02 store
> > functions would be:
> >
> > umwait_cpu_online()
> > {
> >         wrmsr(MSR_IA32_UMWAIT_CONTROL, READ_ONCE(umwait_control_cached), 0);
> >         return 0;
> > }
> >
> > enable_c02_store()
> > {
> >        mutex_lock(&umwait_lock);
> >        umwait_control_c02 = (u32)!c02_enabled;
> >        WRITE_ONCE(umwait_control_cached, 2 | get_umwait_control_max_time());
> >        on_each_cpu(umwait_control_msr_update, NULL, 1);
> >        mutex_unlock(&umwait_lock);
> > }
> >
> > Then suppose umwait_control_cached = 100000 initially and only CPU0 is
> > running. Admin change bit 0 in MSR from 0 to 1 to disable C0.2 and is
> > onlining CPU1 in the same time:
> >
> > 1. On CPU1, read umwait_control_cached to eax as 100000 in
> > umwait_cpu_online()
> > 2. On CPU0, write 100001 to umwait_control_cached in enable_c02_store()
> > 3. On CPU1, wrmsr with eax=100000 in umwaint_cpu_online()
> > 4. On CPU0, wrmsr with 100001 in enabled_c02_store()
> >
> > The result is CPU0 and CPU1 have different MSR values.
> 
> Yes, but only transiently, because you didn't finish your example.
> 
> Step 5: enable_c02_store() does on_each_cpu(), and CPU 1 gets updated.

There is no sync on wrmsr on CPU0 and CPU1. So a better sequence to
describe the problem is changing the order of wrmsr:

1. On CPU1, read umwait_control_cached to eax as 100000 in
umwait_cpu_online()
2. On CPU0, write 100001 to umwait_control_cached in enable_c02_store()
3. On CPU0, wrmsr with 100001 in on_each_cpu() in enabled_c02_store()
4. On CPU1, wrmsr with eax=100000 in umwaint_cpu_online()

So CPU1 and CPU0 have different MSR values. This won't be transient.

So we do need the mutex as in the current patch, right?

Thanks.

-Fenghua

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