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

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Date:   Thu, 13 Jun 2019 21:00:56 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
cc:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        Prasanna Panchamukhi <panchamukhi@...sta.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Cathy Avery <cavery@...hat.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Sasha Levin <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        devel@...uxdriverproject.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment
 vector

On Wed, 12 Jun 2019, Vitaly Kuznetsov wrote:
> Dmitry Safonov <dima@...sta.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.

Thanks,

	tglx