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

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Message-ID: <32ba2406-bc51-01e0-f78e-27eb068cf85a@intel.com>
Date:   Tue, 11 May 2021 09:16:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.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 9:09 AM, Sean Christopherson wrote:
>>> Why does this not result in a #UD if the instruction is disabled by
>>> SEAM? How is it possible to execute a disabled instruction (one
>>> precluded by CPUID) to the point where it triggers #VE instead of #UD?
>> This is actually a vestige of VMX.  It's quite possible toady to have a
>> feature which isn't enumerated in CPUID which still exists and "works"
>> in the silicon.
> No, virtualization holes are something else entirely.  

I think the bigger point is that *CPUID* doesn't enable or disable
instructions in and of itself.

It can *reflect* enabling (like OSPKE), but nothing is actually enabled
or disabled via CPUID.