diff options
| -rw-r--r-- | arch/x86/boot/compressed/head_64.S | 69 |
1 files changed, 53 insertions, 16 deletions
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 959ca07a58d9..fca012baba19 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -307,11 +307,27 @@ ENTRY(startup_64) /* * At this point we are in long mode with 4-level paging enabled, - * but we want to enable 5-level paging. + * but we might want to enable 5-level paging or vice versa. * - * The problem is that we cannot do it directly. Setting LA57 in - * long mode would trigger #GP. So we need to switch off long mode - * first. + * The problem is that we cannot do it directly. Setting or clearing + * CR4.LA57 in long mode would trigger #GP. So we need to switch off + * long mode and paging first. + * + * We also need a trampoline in lower memory to switch over from + * 4- to 5-level paging for cases when the bootloader puts the kernel + * above 4G, but didn't enable 5-level paging for us. + * + * The same trampoline can be used to switch from 5- to 4-level paging + * mode, like when starting 4-level paging kernel via kexec() when + * original kernel worked in 5-level paging mode. + * + * For the trampoline, we need the top page table to reside in lower + * memory as we don't have a way to load 64-bit values into CR3 in + * 32-bit mode. + * + * We go though the trampoline even if we don't have to: if we're + * already in a desired paging mode. This way the trampoline code gets + * tested on every boot. */ /* Make sure we have GDT with 32-bit code segment */ @@ -336,13 +352,18 @@ ENTRY(startup_64) /* Save the trampoline address in RCX */ movq %rax, %rcx + /* + * Load the address of trampoline_return() into RDI. + * It will be used by the trampoline to return to the main code. + */ + leaq trampoline_return(%rip), %rdi /* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */ pushq $__KERNEL32_CS - leaq compatible_mode(%rip), %rax + leaq TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax pushq %rax lretq -lvl5: +trampoline_return: /* Restore the stack, the 32-bit trampoline uses its own stack */ leaq boot_stack_end(%rbx), %rsp @@ -492,8 +513,14 @@ relocated: jmp *%rax .code32 +/* + * This is the 32-bit trampoline that will be copied over to low memory. + * + * RDI contains the return address (might be above 4G). + * ECX contains the base address of the trampoline memory. + * Non zero RDX on return means we need to enable 5-level paging. + */ ENTRY(trampoline_32bit_src) -compatible_mode: /* Set up data and stack segments */ movl $__KERNEL_DS, %eax movl %eax, %ds @@ -534,24 +561,34 @@ compatible_mode: 1: movl %eax, %cr4 - /* Calculate address we are running at */ - call 1f -1: popl %edi - subl $1b, %edi + /* Calculate address of paging_enabled() once we are executing in the trampoline */ + leal paging_enabled - trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_OFFSET(%ecx), %eax - /* Prepare stack for far return to Long Mode */ + /* Prepare the stack for far return to Long Mode */ pushl $__KERNEL_CS - leal lvl5(%edi), %eax - push %eax + pushl %eax - /* Enable paging back */ + /* Enable paging again */ movl $(X86_CR0_PG | X86_CR0_PE), %eax movl %eax, %cr0 lret + .code64 +paging_enabled: + /* Return from the trampoline */ + jmp *%rdi + + /* + * The trampoline code has a size limit. + * Make sure we fail to compile if the trampoline code grows + * beyond TRAMPOLINE_32BIT_CODE_SIZE bytes. + */ + .org trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_SIZE + + .code32 no_longmode: - /* This isn't an x86-64 CPU so hang */ + /* This isn't an x86-64 CPU, so hang intentionally, we cannot continue */ 1: hlt jmp 1b |