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

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Message-ID: <c494ea025188c6b1dcf7ef97a49fcd1cf2dab501.camel@intel.com>
Date: Mon, 21 Jul 2025 23:42:22 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "hpa@...or.com" <hpa@...or.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "peterz@...radead.org" <peterz@...radead.org>,
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	<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>,
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	"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 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.