linux-kernel - Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

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Date:   Fri, 24 Sep 2021 15:37:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

On Thu, Sep 23, 2021 at 10:14:42AM -0700, Luck, Tony wrote:
> On Wed, Sep 22, 2021 at 11:07:22PM +0200, Peter Zijlstra wrote:
> > On Mon, Sep 20, 2021 at 07:23:45PM +0000, Fenghua Yu wrote:
> > > @@ -538,6 +547,9 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
> > >  
> > >  	cond_local_irq_enable(regs);
> > >  
> > > +	if (user_mode(regs) && fixup_pasid_exception())
> > > +		goto exit;
> > > +
> 
> > So you're eating any random #GP that might or might not be PASID
> > related. And all that witout a comment... Enlighten?
> 
> This is moderately well commented inside the fixup_pasid_exception()
> function. Another copy of the comments here at the call-site seems
> overkill.

+static bool fixup_pasid_exception(void)
+{
+       if (!cpu_feature_enabled(X86_FEATURE_ENQCMD))
+               return false;
+
+       return __fixup_pasid_exception();
+}

/me goes looking for comments in that function, lemme get out the
electron microscope, because I can't seem to spot them with the naked
eye.