Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Dmitry Safonov <dima@arista.com>, Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	"Prasanna Panchamukhi" <panchamukhi@arista.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Borislav Petkov" <bp@alien8.de>,
	"Cathy Avery" <cavery@redhat.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
	"Mohammed Gamal" <mmorsy@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Roman Kagan" <rkagan@virtuozzo.com>,
	"Sasha Levin" <sashal@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	devel@linuxdriverproject.org, kvm@vger.kernel.org,
	linux-hyperv@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector
Date: Fri, 14 Jun 2019 23:36:09 +0200	[thread overview]
Message-ID: <87sgsb6e9i.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <cb9e1645-98c2-4341-d6da-4effa4f57fb1@arista.com>

Dmitry Safonov <dima@arista.com> writes:

> On 6/14/19 11:08 AM, Vitaly Kuznetsov wrote:
>> Peter Zijlstra <peterz@infradead.org> writes:
>> 
>>> @@ -182,7 +182,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
>>>  	struct hv_reenlightenment_control re_ctrl = {
>>>  		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
>>>  		.enabled = 1,
>>> -		.target_vp = hv_vp_index[smp_processor_id()]
>>> +		.target_vp = hv_vp_index[raw_smp_processor_id()]
>>>  	};
>>>  	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
>>>  
>> 
>> Yes, this should do, thanks! I'd also suggest to leave a comment like
>> 	/* 
>>          * This function can get preemted and migrate to a different CPU
>> 	 * but this doesn't matter. We just need to assign
>> 	 * reenlightenment notification to some online CPU. In case this
>>          * CPU goes offline, hv_cpu_die() will re-assign it to some
>>  	 * other online CPU.
>> 	 */
>
> What if the cpu goes down just before wrmsrl()?
> I mean, hv_cpu_die() will reassign another cpu, but this thread will be
> resumed on some other cpu and will write cpu number which is at that
> moment already down?
>

Right you are, we need to guarantee wrmsr() happens before the CPU gets
a chance to go offline: we don't save the cpu number anywhere, we just
read it with rdmsr() in hv_cpu_die().

>
> And I presume it's guaranteed that during hv_cpu_die() no other cpu may
> go down:
> :	new_cpu = cpumask_any_but(cpu_online_mask, cpu);
> :	re_ctrl.target_vp = hv_vp_index[new_cpu];
> :	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));

I *think* I got convinced that CPUs don't go offline simultaneously when
I was writing this.

-- 
Vitaly