linux-kernel - Re: [RFC v2 16/32] x86/tdx: Handle MWAIT, MONITOR and WBINVD

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Message-ID: <CAPcyv4gEROpgvf+3Drgso1O6ENQ=2xBoKHqC6d4fWvdDNVSNjA@mail.gmail.com>
Date:   Tue, 11 May 2021 08:50:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 16/32] x86/tdx: Handle MWAIT, MONITOR and WBINVD

On Tue, May 11, 2021 at 8:45 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 5/11/21 8:37 AM, Dan Williams wrote:
> >> I disagree. We already spent a lot of cycles on this. WBINVD makes never
> >> sense in current TDX and all the code will be disabled.
> > Why not just drop the patch if it continues to cause people to spend
> > cycles on it and it addresses a problem that will never happen?
>
> If someone calls WBINVD, we have a bug.  Not a little bug, either.  It
> probably means there's some horribly confused kernel code that's now
> facing broken cache coherency.  To me, it's a textbook place to use
> BUG_ON().
>
> This also doesn't "address" the problem, it just helps produce a more
> coherent warning message.  It's why we have OOPS messages in the page
> fault handler: it never makes any sense to dereference a NULL pointer,
> yet we have code to make debugging them easier.  It's well worth the ~20
> lines of code that this costs us for ease of debugging.

The 'default' case in this 'switch' prints the exit reason and faults,
can't that also trigger a backtrace that dumps the exception stack and
the faulting instruction? In other words shouldn't this just fail with
a common way to provide better debug on any unhandled #VE and not try
to continue running past something that "can't" happen?