linux-kernel - RE: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED

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Message-ID: <BN6PR1101MB2161407D45208C69D32164F0A8EA9@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Tue, 20 Dec 2022 18:47:53 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED_RSP0 during task
 switch

> > --- 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_struct
> *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);
> 
> Urgh, I'm assuming this is a *fast* MSR ?

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

Xin