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SubjectRe: [PATCHv2, RFC] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

* Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:

> This patch addresses shortcoming in current boot process on machines
> that supports 5-level paging.
>
> If bootloader enables 64-bit mode with 4-level paging, we need to
> switch over to 5-level paging. The switching requires disabling paging.
> It works fine if kernel itself is loaded below 4G.
>
> If bootloader put the kernel above 4G (not sure if anybody does this),
> we would loose control as soon as paging is disabled as code becomes
> unreachable.
>
> This patch implements trampoline in lower memory to handle this
> situation.
>
> 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.
>
> place_trampoline() would choose an address for the trampoline page.
> The implementation is based on reserve_bios_regions(). We take a page
> next to end of lowmem.
>
> We only need the page for very short time, until main kernel image
> setup its own page tables.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> arch/x86/boot/compressed/head_64.S | 87 ++++++++++++++++++++++++++------------
> arch/x86/boot/compressed/misc.c | 25 +++++++++++
> 2 files changed, 84 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index cefe4958fda9..961c72755986 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -288,8 +288,23 @@ 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.
> + *
> + * 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.
> + *
> + * The same page can be used to place both trampoline code and top level page
> + * table. place_trampoline() will find suitable place for the trampoline page.
> + * Code will be placed with offset 0x100 from beginning of the page.
> + */
> +#define LVL5_TRAMPOLINE_CODE 0x100
> +
> /* Preserve RBX across CPUID */
> - movq %rbx, %r8
> + movq %rbx, %r15
>
> /* Check if leaf 7 is supported */
> xorl %eax, %eax
> @@ -307,9 +322,6 @@ ENTRY(startup_64)
> andl $(1 << 16), %ecx
> jz lvl5
>
> - /* Restore RBX */
> - movq %r8, %rbx
> -
> /* Check if 5-level paging has already been enabled */
> movq %cr4, %rax
> testl $X86_CR4_LA57, %eax
> @@ -323,34 +335,53 @@ 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
> + /*
> + * Find sitable place for trampoline.
> + * The address will be stored in RBX.
> + */
> + call place_trampoline
> + movq %rax, %rbx
> +
> + /* Preserve RSI, to be used by movsb below */
> + movq %rsi, %r14
> +
> + /* Copy trampoline code in place */
> + leaq lvl5_trampoline_src(%rip), %rsi
> + leaq LVL5_TRAMPOLINE_CODE(%rbx), %rdi
> + movq $(lvl5_trampoline_end - lvl5_trampoline_src), %rcx
> + rep movsb
> +
> + /* Restore RSI */
> + movq %r14, %rsi

Yeah, so first most of this code should be moved from assembly to C. Any reason
why that cannot be done?

Cleanups like that are a precondition to adding this patch or other 5-level
paging complications like the dynamic boot time switching.

Thanks,

Ingo

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