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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 31 Jul 2020 18:28:37 -0700
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Subject: Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Joerg Roedel <joro@8bytes.org>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        H Peter Anvin <hpa@zytor.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Christoph Hellwig <hch@infradead.org>,
        Ashok Raj <ashok.raj@intel.com>,
        Jacob Jun Pan <jacob.jun.pan@intel.com>,
        Dave Jiang <dave.jiang@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 <iommu@lists.linux-foundation.org>,
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On Mon, Jul 13, 2020 at 4:48 PM Fenghua Yu <fenghua.yu@intel.com> wrote:
>
> A #GP fault is generated when ENQCMD instruction is executed without
> a valid PASID value programmed in the current thread's PASID MSR. The
> #GP fault handler will initialize the MSR if a PASID has been allocated
> for this process.

Let's take a step back here.  Why are we trying to avoid IPIs?  If you
call munmap(), you IPI other CPUs running tasks in the current mm.  If
you do perf_event_open() and thus acquire RDPMC permission, you IPI
other CPUs running tasks in the current mm.  If you call modify_ldt(),
you IPI other CPUs running tasks in the current mm.  These events can
all happen more than once per process.

Now we have ENQCMD.  An mm can be assigned a PASID *once* in the model
that these patches support.  Why not just send an IPI using
essentially identical code to the LDT sync or the CR4.PCE sync?

--Andy