linux-kernel - Re: [PATCHv5 06/19] x86/boot/compressed/64: Detect and handle 5-level paging at boot-time

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Date:   Mon, 4 Sep 2017 16:02:42 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 06/19] x86/boot/compressed/64: Detect and handle
 5-level paging at boot-time

On Sun, Aug 27, 2017 at 02:29:26PM +0300, Cyrill Gorcunov wrote:
> On Mon, Aug 21, 2017 at 06:29:03PM +0300, Kirill A. Shutemov wrote:
> > This patch prepare decompression code to boot-time switching between 4-
> > and 5-level paging.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/boot/compressed/head_64.S | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> > 
> > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> > index fbf4c32d0b62..2e362aea3319 100644
> > --- a/arch/x86/boot/compressed/head_64.S
> > +++ b/arch/x86/boot/compressed/head_64.S
> > @@ -347,6 +347,28 @@ preferred_addr:
> >  	leaq	boot_stack_end(%rbx), %rsp
> >  
> >  #ifdef CONFIG_X86_5LEVEL
> > +	/* Preserve rbx across cpuid */
> > +	movq	%rbx, %r8
> > +
> > +	/* Check if leaf 7 is supported */
> > +	movl	$0, %eax
> 
> Use xor instead, it should be shorter
> 
> > +	cpuid
> > +	cmpl	$7, %eax
> > +	jb	lvl5
> > +
> > +	/*
> > +	 * Check if la57 is supported.
> > +	 * The feature is enumerated with CPUID.(EAX=07H, ECX=0):ECX[bit 16]
> > +	 */
> > +	movl	$7, %eax
> > +	movl	$0, %ecx
> 
> same

Thanks. I'll update it for the next re-spin.

-- 
 Kirill A. Shutemov