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

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Date:   Mon, 29 Mar 2021 15:09:51 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.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 3:02 PM, Dave Hansen wrote:
> 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?
We will be reaching this point only if something is buggy in kernel. I am
not sure about impact of this buggy state. But MONITOR instruction by
itself, should not cause 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.
This change is part of the same patch.
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer