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

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Message-ID: <Z_7aeWAxBSdU6cvc@google.com>
Date: Tue, 15 Apr 2025 15:15:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Ross Philipson <ross.philipson@...cle.com>, 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, davem@...emloft.net, corbet@....net, 
	ebiederm@...ssion.com, 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 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".