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Date:   Mon, 15 Jun 2020 20:32:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Raj, Ashok" <ashok.raj@intel.com>
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Frederic Barrat <fbarrat@linux.ibm.com>,
        Andrew Donnellan <ajd@linux.ibm.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Jacob Jun Pan <jacob.jun.pan@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Sohil Mehta <sohil.mehta@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, iommu@lists.linux-foundation.org,
        amd-gfx <amd-gfx@lists.freedesktop.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID
Message-ID: <20200615183204.GE2531@hirez.programming.kicks-ass.net>
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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?