linux-kernel - Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID

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Date:   Mon, 3 Aug 2020 10:34:13 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Tony Luck <tony.luck@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...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 <iommu@...ts.linux-foundation.org>,
        amd-gfx <amd-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH v6 12/12] x86/traps: Fix up invalid PASID

On 8/3/20 10:16 AM, Andy Lutomirski wrote:
> - TILE: genuinely per-thread, but it's expensive so it's
> lazy-loadable.  But the lazy-load mechanism reuses #NM, and it's not
> fully disambiguated from the other use of #NM.  So it sort of works,
> but it's gross.

For those playing along at home, there's a new whitepaper out from Intel
about some new CPU features which are going to be fun:

> https://software.intel.com/content/dam/develop/public/us/en/documents/architecture-instruction-set-extensions-programming-reference.pdf

Which part were you worried about?  I thought it was fully disambuguated
from this:

> When XFD causes an instruction to generate #NM, the processor loads
> the IA32_XFD_ERR MSR to identify the disabled state component(s).
> Specifically, the MSR is loaded with the logical AND of the IA32_XFD
> MSR and the bitmap corresponding to the state components required by
> the faulting instruction.
> 
> Device-not-available exceptions that are not due to XFD — those 
> resulting from setting CR0.TS to 1 — do not modify the IA32_XFD_ERR
> MSR.

So if you always make sure to *clear* IA32_XFD_ERR after handing and XFD
exception, any #NM's with a clear IA32_XFD_ERR are from "legacy"
CR0.TS=1.  Any bits set in IA32_XFD_ERR mean a new-style XFD exception.

Am I missing something?