linux-kernel - Re: [PATCH v13 09/19] x86: Secure Launch kernel early boot stub

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Message-ID: <981894aa-0db0-42e7-a18f-f9d73e75010d@oracle.com>
Date: Tue, 15 Apr 2025 15:16:45 -0700
From: ross.philipson@...cle.com
To: Sean Christopherson <seanjc@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, kexec@...ts.infradead.org,
        linux-efi@...r.kernel.org, iommu@...ts.linux.dev,
        dpsmith@...rtussolutions.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, dave.hansen@...ux.intel.com,
        ardb@...nel.org, mjg59@...f.ucam.org,
        James.Bottomley@...senpartnership.com, peterhuewe@....de,
        jarkko@...nel.org, jgg@...pe.ca, luto@...capital.net,
        nivedita@...m.mit.edu, herbert@...dor.apana.org.au,
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        dwmw2@...radead.org, baolu.lu@...ux.intel.com,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v13 09/19] x86: Secure Launch kernel early boot stub

On 4/15/25 3:15 PM, Sean Christopherson wrote:
> On Tue, Apr 15, 2025, Andrew Cooper wrote:
>> On 11/04/2025 10:40 pm, Sean Christopherson wrote:
>>> On Thu, Apr 10, 2025, Ross Philipson wrote:
>>>> +	 * instruction can return for a number of reasons. Test to see if it returned
>>>> +	 * because the monitor was written to.
>>>> +	 */
>>>> +	monitor
>>>> +
>>>> +1:
>>>> +	mfence
>>>> +	mwait
>>>> +	movl	(%eax), %edx
>>> Why load the value into EDX?  At a glance, the value is never consumed.
>>>
>>>> +	testl	%edx, %edx
>>>> +	jz	1b
>>> This usage of MONITOR/MWAIT is flawed.  The monitor needs to be re-armed in each
>>> loop, otherwise mwait will be a glorified nop.
>>>
>>> More importantly, the exit condition needs to be checked before monitor,
>>
>> after monitor and before mwait.
> 
> Gah, yeah, I mean to say "before mwait".

Thanks Sean and Andy, I will fix this.

Ross