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

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Date:   Mon, 3 Aug 2020 08:12:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 Mon, Aug 3, 2020 at 8:03 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 7/31/20 4:34 PM, Andy Lutomirski wrote:
> >> Thomas suggested to provide a reason for the #GP caused by executing ENQCMD
> >> without a valid PASID value programmed. #GP error codes are 16 bits and all
> >> 16 bits are taken. Refer to SDM Vol 3, Chapter 16.13 for details. The other
> >> choice was to reflect the error code in an MSR. ENQCMD can also cause #GP
> >> when loading from the source operand, so its not fully comprehending all
> >> the reasons. Rather than special case the ENQCMD, in future Intel may
> >> choose a different fault mechanism for such cases if recovery is needed on
> >> #GP.
> > Decoding the user instruction is ugly and sets a bad architecture
> > precedent, but we already do it in #GP for UMIP.  So I'm unconvinced.
>
> I'll try to do one more bit of convincing. :)
>
> In the end, we need a way to figure out if the #GP was from a known "OK"
> source that we can fix up.  You're right that we could fire up the
> instruction decoder to help answer that question.  But, it (also)
> doesn't easily yield a perfect answer as to the source of the #GP, it
> always involves a user copy, and it's a larger code impact than what
> we've got.
>
> I think I went and looked at fixup_umip_exception(), and compared it to
> the alternative which is essentially just these three lines of code:
>
> > +     /*
> > +      * If the current task already has a valid PASID in the MSR,
> > +      * the #GP must be for some other reason.
> > +      */
> > +     if (current->has_valid_pasid)
> > +             return false;
> ...> +  /* Now the current task has a valid PASID in the MSR. */
> > +     current->has_valid_pasid = 1;
>
> and *I* was convinced that instruction decoding wasn't worth it.
>
> There's a lot of stuff that fixup_umip_exception() does which we don't
> have to duplicate, but it's going to be really hard to get it anywhere
> near as compact as what we've got.
>

I could easily be convinced that the PASID fixup is so trivial and so
obviously free of misfiring in a way that causes an infinite loop that
this code is fine.  But I think we first need to answer the bigger
question of why we're doing a lazy fixup in the first place.

--Andy