linux-kernel - Re: [PATCH v2 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

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Message-ID: <d4fe4a10-7c29-5f26-25d2-b23369c38f3e@intel.com>
Date:   Mon, 29 Mar 2021 15:02:15 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD

On 3/29/21 2:55 PM, Kuppuswamy, Sathyanarayanan wrote:
>>
>> MONITOR is a privileged instruction, right?  So we can only end up in
>> here if the kernel screws up and isn't reading CPUID correctly, right?
>>
>> That dosen't seem to me like something we want to suppress.  This needs
>> a warning, at least.  I assume that having a MONITOR instruction
>> immediately return doesn't do any harm.
> Agree. Since we are not supposed to come here, I will use BUG.

"This is unexpected" is a WARN()able offense.

"This is unexpected and might be corrupting data" is where we want to
use BUG().

Does broken MONITOR risk data corruption?

>>> +    case EXIT_REASON_MWAIT_INSTRUCTION:
>>> +        /* MWAIT is supressed, not supposed to reach here. */
>>> +        WARN(1, "MWAIT unexpected #VE Exception\n");
>>> +        return -EFAULT;
>>
>> How is MWAIT "supppressed"?
> I am clearing the MWAIT feature flag in early init code. We should also
> disable this feature in firmware. setup_clear_cpu_cap(X86_FEATURE_MWAIT);

I'd be more explicit about that.  Maybe even reference the code that
clears the X86_FEATURE.