linux-kernel - Re: [tip:x86/boot] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

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Message-ID: <20180213113219.rn5xdilh7j3qvfme@gmail.com>
Date:   Tue, 13 Feb 2018 12:32:19 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrei Vagin <avagin@...tuozzo.com>,
        tip-bot for Jacob Shin <tipbot@...or.com>,
        kirill.shutemov@...ux.intel.com, linux-tip-commits@...r.kernel.org,
        tglx@...utronix.de, willy@...radead.org, gorcunov@...nvz.org,
        bp@...e.de, peterz@...radead.org, torvalds@...ux-foundation.org,
        hpa@...or.com, linux-kernel@...r.kernel.org, luto@...capital.net
Subject: Re: [tip:x86/boot] x86/boot/compressed/64: Handle 5-level paging
 boot if kernel is above 4G


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> On Tue, Feb 13, 2018 at 10:43:56AM +0100, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill@...temov.name> wrote:
> > 
> > > On Tue, Feb 13, 2018 at 12:41:22AM -0800, Andrei Vagin wrote:
> > > > On Tue, Feb 13, 2018 at 11:08:16AM +0300, Kirill A. Shutemov wrote:
> > > > > On Mon, Feb 12, 2018 at 10:51:56PM -0800, Andrei Vagin wrote:
> > > > > > Hi Kirill,
> > > > > > 
> > > > > > Something is wrong in this patch.
> > > 
> > > Could you please check if this makes a difference?
> > > 
> > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> > > index 70b30f2bc9e0..99a0e7993252 100644
> > > --- a/arch/x86/boot/compressed/head_64.S
> > > +++ b/arch/x86/boot/compressed/head_64.S
> > > @@ -332,7 +332,7 @@ ENTRY(startup_64)
> > >  
> > >  	/* Make sure we have GDT with 32-bit code segment */
> > >  	leaq	gdt(%rip), %rax
> > > -	movl	%eax, gdt64+2(%rip)
> > > +	movq	%rax, gdt64+2(%rip)
> > >  	lgdt	gdt64(%rip)
> > 
> > There's another suspicious looking pattern as well:
> > 
> >         leaq    startup_32(%rip), %rax
> >         movl    %eax, BP_code32_start(%rsi)
> > ...
> >         movl    BP_code32_start(%esi), %eax
> >         leaq    startup_64(%rax), %rax
> > ...
> 
> code32_start is 4-byte field as described in the boot protocol, so the
> truncation is intentional I think.

Ok - and I guess the fact that the field includes '32' is documentation enough 
that this is expected.

Thanks,

	Ingo