Re: [Xen-devel] [PATCH] x86/xen: zero MSR_IA32_SPEC_CTRL before suspend

[Juergen Gross <jgross@xxxxxxxx>]

On 14/03/18 09:48, Jan Beulich wrote:
>>>> On 26.02.18 at 15:08, <jgross@xxxxxxxx> wrote:
>> @@ -35,6 +40,9 @@ void xen_arch_post_suspend(int cancelled)
>>  
>>  static void xen_vcpu_notify_restore(void *data)
>>  {
>> +    if (xen_pv_domain() && boot_cpu_has(X86_FEATURE_SPEC_CTRL))
>> +            wrmsrl(MSR_IA32_SPEC_CTRL, this_cpu_read(spec_ctrl));
>> +
>>      /* Boot processor notified via generic timekeeping_resume() */
>>      if (smp_processor_id() == 0)
>>              return;
>> @@ -44,7 +52,15 @@ static void xen_vcpu_notify_restore(void *data)
>>  
>>  static void xen_vcpu_notify_suspend(void *data)
>>  {
>> +    u64 tmp;
>> +
>>      tick_suspend_local();
>> +
>> +    if (xen_pv_domain() && boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
>> +            rdmsrl(MSR_IA32_SPEC_CTRL, tmp);
>> +            this_cpu_write(spec_ctrl, tmp);
>> +            wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>> +    }
>>  }
> 
> While investigating ways how to do something similar on our old,
> non-pvops kernels I've started wondering if this solution is actually
> correct in all cases. Of course discussing this is complicated by the
> fact that the change there might be a conflict with hasn't landed
> in Linus'es tree yet (see e.g.
> https://patchwork.kernel.org/patch/10153843/ for an upstream
> submission; I haven't been able to find any discussion on that
> patch or why it isn't upstream yet), but we have it in our various
> branches. The potential problem I'm seeing is with the clearing
> and re-setting of SPEC_CTRL around CPUs going idle. While the
> active CPU could have preemption disabled (if that isn't the case
> already), the passive CPUs are - afaict - neither under full control
> of drivers/xen/manage.c:do_suspend() nor excluded yet from
> any further scheduling activity. Hence with code like this (taken
> from one of our branches)
> 
> static void mwait_idle(void)
> {
>       if (!current_set_polling_and_test()) {
>               trace_cpu_idle_rcuidle(1, smp_processor_id());
>               if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
>                       smp_mb(); /* quirk */
>                       clflush((void *)&current_thread_info()->flags);
>                       smp_mb(); /* quirk */
>               }
> 
>               x86_disable_ibrs();
> 
>               __monitor((void *)&current_thread_info()->flags, 0, 0);
>               if (!need_resched())
>                       __sti_mwait(0, 0);
>               else
>                       local_irq_enable();
> 
>               x86_enable_ibrs();
>               ...
> 
> the MSR might get set to non-zero again after having been
> cleared by the code your patch adds. I therefore think that the
> only race free solution would be to do the clearing from
> stop-machine context. But maybe I'm overlooking something.

Currently and with the above mentioned patch there is no problem: Xen pv
guests always use default_idle(), so mwait_idle() eventually playing
with MSR_IA32_SPEC_CTRL won't affect us.

In order to ensure that won't change in future default_idle() should
never modify MSR_IA32_SPEC_CTRL. In case something like that would be
required we should rather add another idle function doing that.


Juergen

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