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

[Dave Hansen]

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?

> 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.

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

>  	/*
> -	 * 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?

>  	/* 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.

>  	push	%eax
>  
>  	/* Enable paging back */
> @@ -525,8 +544,15 @@ compatible_mode:
>  	movl	%eax, %cr0
>  
>  	lret
> +
> +	.code64
> +lvl5_enabled:
> +	/* Return from trampoline */
> +	jmp	*%rdi
> +lvl5_trampoline_end:
>  #endif
>  
> +	.code32
>  no_longmode:
>  	/* This isn't an x86-64 CPU so hang */
>  1:
> @@ -584,7 +610,3 @@ boot_stack_end:
>  	.balign 4096
>  pgtable:
>  	.fill BOOT_PGT_SIZE, 1, 0
> -#ifdef CONFIG_X86_5LEVEL
> -lvl5_pgtable:
> -	.fill PAGE_SIZE, 1, 0
> -#endif
>