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Subject: Re: [RFC v2 16/32] x86/tdx: Handle MWAIT, MONITOR and WBINVD
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <cover.1619458733.git.sathyanarayanan.kuppuswamy@linux.intel.com>
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 <04c3922a-36e2-bf07-e5fd-0d2eebda250b@linux.intel.com>
From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <bd43b967-26ed-d38e-e237-3a274f6b276e@linux.intel.com>
Date:   Mon, 10 May 2021 19:51:26 -0700
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On 5/10/2021 7:44 PM, Kuppuswamy, Sathyanarayanan wrote:
>
>
> On 5/10/21 7:17 PM, Andi Kleen wrote:
>>>> To prevent TD guest from using MWAIT/MONITOR instructions,
>>>> support for these instructions are already disabled by TDX
>>>> module (SEAM). So CPUID flags for these instructions should
>>>> be in disabled state.
>>> Why does this not result in a #UD if the instruction is disabled by
>>> SEAM?
>>
>> It's just the TDX module (SEAM is the execution mode used by the TDX 
>> module)
>
> If it is disabled by the TDX Module, we should never execute it. But 
> for some
> reason, if we still come across this instruction (buggy TDX module?), 
> we add
> appropriate warning in  #VE handler.

I think the only case where it could happen is if the kernel jumps to a 
random address due to a bug and the destination happens to be these 
instruction bytes. Of course it is exceedingly unlikely.

Or we make some mistake, but that's hopefully fixed quickly.


-Andi