linux-kernel - Re: [PATCH v4 1/7] x86/kexec: Consolidate relocate

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Message-ID: <38C8C851-8533-4F1E-B047-5DD55C123CD1@zytor.com>
Date: Mon, 21 Jul 2025 16:05:04 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Huang, Kai" <kai.huang@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>
CC: "Gao, Chao" <chao.gao@...el.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>, "kas@...nel.org" <kas@...nel.org>,
        "sagis@...gle.com" <sagis@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "ashish.kalra@....com" <ashish.kalra@....com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v4 1/7] x86/kexec: Consolidate relocate_kernel() function parameters

On July 21, 2025 2:36:48 PM PDT, "Huang, Kai" <kai.huang@...el.com> wrote:
>On Mon, 2025-07-21 at 16:27 -0500, Tom Lendacky wrote:
>> > > > @@ -204,7 +202,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>> > > >   	 * entries that will conflict with the now unencrypted memory
>> > > >   	 * used by kexec. Flush the caches before copying the kernel.
>> > > >   	 */
>> > > > -	testq	%r8, %r8
>> > > > +	testq	$RELOC_KERNEL_HOST_MEM_ACTIVE, %r11
>> > > 
>> > > Hmmm... can't both bits be set at the same time? If so, then this will
>> > > fail. This should be doing bit tests now.
>> > 
>> > TEST instruction performs logical AND of the two operands, therefore the
>> > above equals to:
>> > 
>> >  	set ZF if "R11 AND BIT(1) == 0".
>> > 
>> > Whether there's any other bits set in R11 doesn't impact the above, right?
>> >   
>> 
>> Doh! My bad, yes, not sure what I was thinking there.
>> 
>
>Np and thanks! I'll address your other comments but I'll see whether Boris
>has any other comments first.
>

You can use testb in this case to save 3 bytes, too.