linux-kernel - Re: [PATCH] x86/xen: zero MSR_IA32_SPEC

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Date:   Wed, 11 Apr 2018 03:15:39 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     <tglx@...utronix.de>, <mingo@...hat.com>,
        "Juergen Gross" <jgross@...e.com>, <hpa@...or.com>
Cc:     <x86@...nel.org>, <xen-devel@...ts.xenproject.org>,
        <boris.ostrovsky@...cle.com>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/xen: zero MSR_IA32_SPEC_CTRL before suspend

>>> On 11.04.18 at 09:08, <jgross@...e.com> wrote:
> On 14/03/18 09:48, Jan Beulich wrote:
>>>>> On 26.02.18 at 15:08, <jgross@...e.com> 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.

It's pretty unclear to me why default_idle() doesn't have this - in
Xen we do it for all idle routines.

> 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.

This looks like a pretty strange/non-obvious requirement, which I
don't think anyone would remember.

Additionally, x86 maintainers: is there a particular reason this (or
any functionally equivalent patch) isn't upstream yet? As indicated
before, I had not been able to find any discussion, and hence I
see no reason why this is a patch we effectively carry privately in
our distro branches (and likely other distros do so too).

Jan