linux-kernel - Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID

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Date:   Mon, 15 Jun 2020 20:32:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Raj, Ashok" <ashok.raj@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Frederic Barrat <fbarrat@...ux.ibm.com>,
        Andrew Donnellan <ajd@...ux.ibm.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, iommu@...ts.linux-foundation.org,
        amd-gfx <amd-gfx@...ts.freedesktop.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID

On Mon, Jun 15, 2020 at 11:19:21AM -0700, Raj, Ashok wrote:
> On Mon, Jun 15, 2020 at 06:03:57PM +0200, Peter Zijlstra wrote:
> > 
> > I don't get why you need a rdmsr here, or why not having one would
> > require a TIF flag. Is that because this MSR is XSAVE/XRSTOR managed?
> > 
> > > > > +	 */
> > > > > +	rdmsrl(MSR_IA32_PASID, pasid_msr);
> > > > > +	if (pasid_msr & MSR_IA32_PASID_VALID)
> > > > > +		return false;
> > > > > +
> > > > > +	/* Fix up the MSR if the MSR doesn't have a valid PASID. */
> > > > > +	wrmsrl(MSR_IA32_PASID, pasid | MSR_IA32_PASID_VALID);
> > 
> > How much more expensive is the wrmsr over the rdmsr? Can't we just
> > unconditionally write the current PASID and call it a day?
> 
> The reason to check the rdmsr() is because we are using a hueristic taking
> GP faults. If we already setup the MSR, but we get it a second time it
> means the reason is something other than PASID_MSR not being set.
> 
> Ideally we should use the TIF_ to track this would be cheaper, but we were
> told those bits aren't easy to give out. 

Why do you need a TIF flag? Why not any other random flag in current?