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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Oct 09, 2017 at 09:54:53AM -0700, Dave Hansen wrote:
> On 10/09/2017 09:09 AM, Kirill A. Shutemov wrote:
> > Apart from trampoline itself we also need place to store top level page
> > table in lower memory as we don't have a way to load 64-bit value into
> > CR3 from 32-bit mode. We only really need 8-bytes there as we only use
> > the very first entry of the page table.
> 
> Oh, and this is why you have to move "lvl5_pgtable" out of the kernel image?

Right. I initialize the new location of top level page table directly.

> > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> > index cefe4958fda9..049a289342bd 100644
> > --- a/arch/x86/boot/compressed/head_64.S
> > +++ b/arch/x86/boot/compressed/head_64.S
> > @@ -288,6 +288,22 @@ ENTRY(startup_64)
> >  	leaq	boot_stack_end(%rbx), %rsp
> >  
> >  #ifdef CONFIG_X86_5LEVEL
> > +/*
> > + * We need trampoline in lower memory switch from 4- to 5-level paging for
> > + * cases when bootloader put kernel above 4G, but didn't enable 5-level paging
> > + * for us.
> > + *
> > + * Here we use MBR memory to store trampoline code.
> > + *
> > + * We also have to have top page table in lower memory as we don't have a way
> > + * to load 64-bit value into CR3 from 32-bit mode. We only need 8-bytes there
> > + * as we only use the very first entry of the page table.
> > + *
> > + * Here we use 0x7000 as top-level page table.
> > + */
> > +#define LVL5_TRAMPOLINE	0x7c00
> > +#define LVL5_PGTABLE	0x7000
> > +
> >  	/* Preserve RBX across CPUID */
> >  	movq	%rbx, %r8
> >  
> > @@ -323,29 +339,37 @@ ENTRY(startup_64)
> >  	 * long mode would trigger #GP. So we need to switch off long mode
> >  	 * first.
> >  	 *
> > -	 * NOTE: This is not going to work if bootloader put us above 4G
> > -	 * limit.
> > +	 * We use trampoline in lower memory to handle situation when
> > +	 * bootloader put the kernel image above 4G.
> >  	 *
> >  	 * The first step is go into compatibility mode.
> >  	 */
> >  
> > -	/* Clear additional page table */
> > -	leaq	lvl5_pgtable(%rbx), %rdi
> > -	xorq	%rax, %rax
> > -	movq	$(PAGE_SIZE/8), %rcx
> > -	rep	stosq
> > +	/* Copy trampoline code in place */
> > +	movq	%rsi, %r9
> > +	leaq	lvl5_trampoline(%rip), %rsi
> > +	movq	$LVL5_TRAMPOLINE, %rdi
> > +	movq	$(lvl5_trampoline_end - lvl5_trampoline), %rcx
> > +	rep	movsb
> > +	movq	%r9, %rsi
> 
> This needs to get more heavily commented, like the use of r9 to stash
> %rsi.  Why do you do that, btw?  I don't see it getting reused at first
> glance.

%rsi holds pointer to real_mode_data. It need to be preserved.

I'll add more comments.

> I think it will also be really nice to differentate "lvl5_trampoline"
> from "LVL5_TRAMPOLINE".  Maybe add "src" and "dst" to them or something.

Makes sense. Thanks.

> >  	/*
> > -	 * Setup current CR3 as the first and only entry in a new top level
> > +	 * Setup current CR3 as the first and the only entry in a new top level
> >  	 * page table.
> >  	 */
> >  	movq	%cr3, %rdi
> >  	leaq	0x7 (%rdi), %rax
> > -	movq	%rax, lvl5_pgtable(%rbx)
> > +	movq	%rax, LVL5_PGTABLE
> > +
> > +	/*
> > +	 * Load address of lvl5 into RDI.
> > +	 * It will be used to return address from trampoline.
> > +	 */
> > +	leaq	lvl5(%rip), %rdi
> 
> Is there a reason to do a 'lea' here instead of just shoving the address
> in directly?  Is this a shorter instruction or something?

This code can be loaded anywhere in memory and we need to calculate
absolute address of the label here.
AFAIK, "lea <label>(%rip), <register>" is idiomatic way to do this.

> >  	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
> >  	pushq	$__KERNEL32_CS
> > -	leaq	compatible_mode(%rip), %rax
> > +	movq	$LVL5_TRAMPOLINE, %rax
> >  	pushq	%rax
> >  	lretq
> >  lvl5:
> > @@ -488,9 +512,9 @@ relocated:
> >   */
> >  	jmp	*%rax
> >  
> > -	.code32
> >  #ifdef CONFIG_X86_5LEVEL
> > -compatible_mode:
> > +	.code32
> > +lvl5_trampoline:
> >  	/* Setup data and stack segments */
> >  	movl	$__KERNEL_DS, %eax
> >  	movl	%eax, %ds
> > @@ -502,7 +526,7 @@ compatible_mode:
> >  	movl	%eax, %cr0
> >  
> >  	/* Point CR3 to 5-level paging */
> > -	leal	lvl5_pgtable(%ebx), %eax
> > +	movl	$LVL5_PGTABLE, %eax
> >  	movl	%eax, %cr3
> >  
> >  	/* Enable PAE and LA57 mode */
> > @@ -510,14 +534,9 @@ compatible_mode:
> >  	orl	$(X86_CR4_PAE | X86_CR4_LA57), %eax
> >  	movl	%eax, %cr4
> >  
> > -	/* Calculate address we are running at */
> > -	call	1f
> > -1:	popl	%edi
> > -	subl	$1b, %edi
> > -
> >  	/* Prepare stack for far return to Long Mode */
> >  	pushl	$__KERNEL_CS
> > -	leal	lvl5(%edi), %eax
> > +	movl	$(lvl5_enabled - lvl5_trampoline + LVL5_TRAMPOLINE), %eax
> 
> This loads the trampoline address of "lvl5_enabled", right?  That'd be
> handy to spell out explicitly.

Yep, will do.

-- 
 Kirill A. Shutemov

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