This patch addresses a shortcoming in current boot process on machines that supports 5-level paging. If a bootloader enables 64-bit mode with 4-level paging, we might need to switch over to 5-level paging. The switching requires the disabling paging. It works fine if kernel itself is loaded below 4G. But if the bootloader put the kernel above 4G (not sure if anybody does this), we would lose control as soon as paging is disabled, because the code becomes unreachable to the CPU. This patch implements a trampoline in lower memory to handle this situation. We only need the memory for a very short time, until the main kernel image sets up own page tables. We go through the trampoline even if we don't have to: if we're already in 5-level paging mode or if we don't need to switch to it. This way the trampoline gets tested on every boot. Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> --- arch/x86/boot/compressed/head_64.S | 102 +++++++++++++++++++++++-------------- 1 file changed, 65 insertions(+), 37 deletions(-) diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 1bcc62a232f6..fa45e801e132 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -33,6 +33,7 @@ #include <asm/processor-flags.h> #include <asm/asm-offsets.h> #include <asm/bootparam.h> +#include "pgtable.h" /* * Locally defined symbols should be marked hidden: @@ -306,11 +307,23 @@ 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. * - * 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 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. + * + * 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 5-level paging mode or if we don't need to switch to + * it. This way the trampoline code gets tested on every boot. */ /* @@ -330,30 +343,21 @@ ENTRY(startup_64) /* Save the trampoline address in RCX */ movq %rax, %rcx - /* Check if we need to enable 5-level paging */ - cmpq $0, %rdx - jz lvl5 - - /* Clear additional page table */ - leaq lvl5_pgtable(%rbx), %rdi - xorq %rax, %rax - movq $(PAGE_SIZE/8), %rcx - rep stosq - /* - * Setup current CR3 as the first and only entry in a new top level - * page table. + * Load the address of trampoline_return() into RDI. + * It will be used by the trampoline to return to the main code. */ - movq %cr3, %rdi - leaq 0x7 (%rdi), %rax - movq %rax, lvl5_pgtable(%rbx) + 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 + andq $~1, %rax /* Clear bit 0: encodes if 5-level paging needed */ + 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 /* Zero EFLAGS */ pushq $0 @@ -491,45 +495,71 @@ 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 movl %eax, %ss + /* Setup new stack */ + leal TRAMPOLINE_32BIT_STACK_END(%ecx), %esp + /* Disable paging */ movl %cr0, %eax btrl $X86_CR0_PG_BIT, %eax movl %eax, %cr0 - /* Point CR3 to 5-level paging */ - leal lvl5_pgtable(%ebx), %eax + /* For 5-level paging, point CR3 to the trampoline's new top level page table */ + cmpl $0, %edx + jz 1f + leal TRAMPOLINE_32BIT_PGTABLE_OFFSET(%ecx), %eax movl %eax, %cr3 +1: - /* Enable PAE and LA57 mode */ + /* Enable PAE and LA57 (if required) paging modes */ movl %cr4, %eax - orl $(X86_CR4_PAE | X86_CR4_LA57), %eax + orl $X86_CR4_PAE, %eax + cmpl $0, %edx + jz 1f + orl $X86_CR4_LA57, %eax +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 @@ -585,5 +615,3 @@ boot_stack_end: .balign 4096 pgtable: .fill BOOT_PGT_SIZE, 1, 0 -lvl5_pgtable: - .fill PAGE_SIZE, 1, 0 -- 2.15.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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