[PATCH 4/4] x86: boot: avoid memory copy if kernel is uncompressed

From: Albert Huang
Date: Mon Jul 25 2022 - 04:40:46 EST


From: "huangjie.albert" <huangjie.albert@xxxxxxxxxxxxx>

1、if kernel is uncompressed. we do not need to relocate
kernel image for decompression

2、if kaslr is disabled, we do not need to do a memory copy
before prase_elf.

Two memory copies can be skipped with this patch. this can
save aboat 20ms during booting.

Signed-off-by: huangjie.albert <huangjie.albert@xxxxxxxxxxxxx>
---
arch/x86/boot/compressed/head_64.S | 8 ++++++--
arch/x86/boot/compressed/misc.c | 22 +++++++++++++++++-----
2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index d33f060900d2..9e7770c7047b 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -398,10 +398,13 @@ SYM_CODE_START(startup_64)
1:

/* Target address to relocate to for decompression */
+#ifdef CONFIG_KERNEL_UNCOMPRESSED
+ movq %rbp, %rbx
+#else
movl BP_init_size(%rsi), %ebx
subl $ rva(_end), %ebx
addq %rbp, %rbx
-
+#endif
/* Set up the stack */
leaq rva(boot_stack_end)(%rbx), %rsp

@@ -522,6 +525,7 @@ trampoline_return:
* Copy the compressed kernel to the end of our buffer
* where decompression in place becomes safe.
*/
+#ifndef CONFIG_KERNEL_UNCOMPRESSED
pushq %rsi
leaq (_bss-8)(%rip), %rsi
leaq rva(_bss-8)(%rbx), %rdi
@@ -531,7 +535,7 @@ trampoline_return:
rep movsq
cld
popq %rsi
-
+#endif
/*
* The GDT may get overwritten either during the copy we just did or
* during extract_kernel below. To avoid any issues, repoint the GDTR
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index c23c0f525d93..d8445562d4e9 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -290,7 +290,7 @@ static inline void handle_relocations(void *output, unsigned long output_len,
{ }
#endif

-static void parse_elf(void *output)
+static void parse_elf(void *output, void *input)
{
#ifdef CONFIG_X86_64
Elf64_Ehdr ehdr;
@@ -302,7 +302,7 @@ static void parse_elf(void *output)
void *dest;
int i;

- memcpy(&ehdr, output, sizeof(ehdr));
+ memcpy(&ehdr, input, sizeof(ehdr));
if (ehdr.e_ident[EI_MAG0] != ELFMAG0 ||
ehdr.e_ident[EI_MAG1] != ELFMAG1 ||
ehdr.e_ident[EI_MAG2] != ELFMAG2 ||
@@ -317,7 +317,7 @@ static void parse_elf(void *output)
if (!phdrs)
error("Failed to allocate space for phdrs");

- memcpy(phdrs, output + ehdr.e_phoff, sizeof(*phdrs) * ehdr.e_phnum);
+ memcpy(phdrs, input + ehdr.e_phoff, sizeof(*phdrs) * ehdr.e_phnum);

for (i = 0; i < ehdr.e_phnum; i++) {
phdr = &phdrs[i];
@@ -334,7 +334,7 @@ static void parse_elf(void *output)
#else
dest = (void *)(phdr->p_paddr);
#endif
- memmove(dest, output + phdr->p_offset, phdr->p_filesz);
+ memmove(dest, input + phdr->p_offset, phdr->p_filesz);
break;
default: /* Ignore other PT_* */ break;
}
@@ -467,9 +467,21 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
#endif

debug_putstr("\nDecompressing Linux... ");
+
+#ifdef CONFIG_KERNEL_UNCOMPRESSED
+ if (cmdline_find_option_bool("nokaslr")) {
+ parse_elf(output, input_data);
+ } else {
+ __decompress(input_data, input_len, NULL, NULL, output, output_len,
+ NULL, error);
+ parse_elf(output, output);
+ }
+#else
__decompress(input_data, input_len, NULL, NULL, output, output_len,
NULL, error);
- parse_elf(output);
+ parse_elf(output, output);
+#endif
+
handle_relocations(output, output_len, virt_addr);
debug_putstr("done.\nBooting the kernel.\n");

--
2.31.1