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

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Message-ID: <D6A63DDD-6A33-4A78-8A3F-2A7D0ACC9902@zytor.com>
Date: Mon, 21 Jul 2025 17:44:11 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Huang, Kai" <kai.huang@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "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>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "ashish.kalra@....com" <ashish.kalra@....com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v4 1/7] x86/kexec: Consolidate relocate_kernel() function parameters

On July 21, 2025 4:42:22 PM PDT, "Huang, Kai" <kai.huang@...el.com> wrote:
>On Mon, 2025-07-21 at 23:29 +0000, Huang, Kai wrote:
>> > 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.
>> 
>> Yeah I can do that, thanks for the info!
>
>I just tried.  I need to do:
>
>	testb	$RELOC_KERNEL_HOST_MEM_ACTIVE, %r11b
>
>in order to compile, otherwise using plain %r11 generates:
>
>arch/x86/kernel/relocate_kernel_64.S:212: Error: `%r11' not allowed with
>`testb'
>
>I'll do some test and if there's no problem I'll switch to use this way,
>assuming it still saves us 3-bytes.
>

That works just fine.