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From:   "Li, Xin3" <xin3.li@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
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        "Christopherson,, Sean" <seanjc@google.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>
Subject: RE: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED_RSP0 during task
 switch
Thread-Topic: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED_RSP0 during
 task switch
Thread-Index: AQHZFEEtGjKidcc/l0em/MMw2NP2fa52h/wAgACVoVA=
Date:   Tue, 20 Dec 2022 18:47:53 +0000
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References: <20221220063658.19271-1-xin3.li@intel.com>
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> > --- a/arch/x86/include/asm/switch_to.h
> > +++ b/arch/x86/include/asm/switch_to.h
> > @@ -71,9 +71,13 @@ static inline void update_task_stack(struct task_str=
uct
> *task)
> >  	else
> >  		this_cpu_write(cpu_tss_rw.x86_tss.sp1, task->thread.sp0);  #else
> > -	/* Xen PV enters the kernel on the thread stack. */
> > -	if (static_cpu_has(X86_FEATURE_XENPV))
> > +	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> > +		wrmsrl(MSR_IA32_FRED_RSP0,
> > +		       task_top_of_stack(task) + TOP_OF_KERNEL_STACK_PADDING);
>=20
> Urgh, I'm assuming this is a *fast* MSR ?

You're right, however that is another feature under development AFAIK.

Xin