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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Dmitry Safonov" <dima@arista.com>,
	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>,
	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 10:06:43 +0200	[thread overview]
Message-ID: <87a7ek7fqk.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1906132059020.1791@nanos.tec.linutronix.de>

Thomas Gleixner <tglx@linutronix.de> writes:

> On Wed, 12 Jun 2019, Vitaly Kuznetsov wrote:
>> Dmitry Safonov <dima@arista.com> writes:
>> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> > index 1608050e9df9..0bdd79ecbff8 100644
>> > --- a/arch/x86/hyperv/hv_init.c
>> > +++ b/arch/x86/hyperv/hv_init.c
>> > @@ -91,7 +91,7 @@ EXPORT_SYMBOL_GPL(hv_max_vp_index);
>> >  static int hv_cpu_init(unsigned int cpu)
>> >  {
>> >  	u64 msr_vp_index;
>> > -	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
>> > +	struct hv_vp_assist_page **hvp = &hv_vp_assist_page[cpu];
>> >  	void **input_arg;
>> >  	struct page *pg;
>> >  
>> > @@ -103,7 +103,7 @@ static int hv_cpu_init(unsigned int cpu)
>> >  
>> >  	hv_get_vp_index(msr_vp_index);
>> >  
>> > -	hv_vp_index[smp_processor_id()] = msr_vp_index;
>> > +	hv_vp_index[cpu] = msr_vp_index;
>> >  
>> >  	if (msr_vp_index > hv_max_vp_index)
>> >  		hv_max_vp_index = msr_vp_index;
>> 
>> The above is unrelated cleanup (as cpu == smp_processor_id() for
>> CPUHP_AP_ONLINE_DYN callbacks), right? As I'm pretty sure these can'd be
>> preempted.
>
> They can be preempted, but they are guaranteed to run on the upcoming CPU,
> i.e. smp_processor_id() is allowed even in preemptible context as the task
> cannot migrate.
>

Ah, right, thanks! The guarantee that they don't migrate should be enough.

-- 
Vitaly