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Subject: Re: [PATCH v2 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD
To:     Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        linux-kernel@vger.kernel.org
References: <91C0F6F1-B8C3-4130-B903-C63CD4B87F3F@amacapital.net>
 <ed7e96ba7a271e36bdfe61ee34c6d61eb78000c5.1616885306.git.sathyanarayanan.kuppuswamy@linux.intel.com>
 <498f078c-5196-a608-7f1a-8425ff056135@intel.com>
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Message-ID: <8e02ce39-f672-e652-b314-418b7ec5f52a@linux.intel.com>
Date:   Mon, 29 Mar 2021 14:55:39 -0700
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On 3/29/21 10:14 AM, Dave Hansen wrote:
> On 3/27/21 3:54 PM, Kuppuswamy Sathyanarayanan wrote:
>> +	/*
>> +	 * Per Guest-Host-Communication Interface (GHCI) for Intel Trust
>> +	 * Domain Extensions (Intel TDX) specification, sec 2.4,
>> +	 * some instructions that unconditionally cause #VE (such as WBINVD,
>> +	 * MONITOR, MWAIT) do not have corresponding TDCALL
>> +	 * [TDG.VP.VMCALL <Instruction>] leaves, since the TD has been designed
>> +	 * with no deterministic way to confirm the result of those operations
>> +	 * performed by the host VMM.  In those cases, the goal is for the TD
>> +	 * #VE handler to increment the RIP appropriately based on the VE
>> +	 * information provided via TDCALL.
>> +	 */

> 
> That's an awfully big comment.  Could you pare it down, please?  Maybe
> focus on the fact that we should never get here and why, rather than
> talking about some silly spec?
I will remove this and add individual one line comment for WBINVD and MONITOR
instructions. Some thing like "Privileged instruction, can only be executed
in ring 0. So raise a BUG.
> 
>> +	case EXIT_REASON_WBINVD:
>> +		pr_warn_once("WBINVD #VE Exception\n");
> 
> I actually think WBINVD in here should oops.  We use it for some really
> important things.  If it can't be executed, and we're depending on it,
> the kernel is in deep, deep trouble.
Agree. I will call BUG().
> 
> I think a noop here is dangerous.
> 
>> +	case EXIT_REASON_MONITOR_INSTRUCTION:
>> +		/* Handle as nops. */
>> +		break;
> 
> 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.
> 
>> +	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);
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer