linux-kernel - RE: [PATCH v5 23/34] x86/fred: update MSR_IA32_FRED

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Date:   Mon, 20 Mar 2023 23:54:31 +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: [PATCH v5 23/34] x86/fred: update MSR_IA32_FRED_RSP0 during task
 switch

> > -	if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > +	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> > +		/*
> > +		 * Will use WRMSRNS/WRMSRLIST for performance once it's
> upstreamed.
> > +		 */
> > +		wrmsrl(MSR_IA32_FRED_RSP0,
> > +		       task_top_of_stack(task) +
> TOP_OF_KERNEL_STACK_PADDING);
> > +	} else if (cpu_feature_enabled(X86_FEATURE_XENPV)) {
> 
> Whee, so hardware will really only ever look at this when RSP0? I don't need to
> worry about exceptions during context switch?

You're right, we don't.

RSP0 is only used in ring3. Exceptions from ring0 just keep using the current
kernel stack unless a higher stack level needs to be used, e.g., RSP3 for #DF.

Thanks!
  Xin