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

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Message-ID: <43e0a5cc-721a-04f1-50b6-b1319da10bac@intel.com>
Date:   Tue, 11 May 2021 08:44:43 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     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 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.