x86/boot/compressed/32: Save the output address instead of recalculating it

In preparation for being able to decompress into a buffer starting at a
different address than startup_32, save the calculated output address
instead of recalculating it later.

We now keep track of three addresses:

	%edx: startup_32 as we were loaded by bootloader
	%ebx: new location of compressed kernel
	%ebp: start of decompression buffer

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200303221205.4048668-2-nivedita@alum.mit.edu
Link: https://lore.kernel.org/r/20200308080859.21568-16-ardb@kernel.org

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 46bbe7a..8941825 100644 (file)
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -75,11 +75,11 @@ SYM_FUNC_START(startup_32)
  */
        leal    (BP_scratch+4)(%esi), %esp
        call    1f
-1:     popl    %ebp
-       subl    $1b, %ebp
+1:     popl    %edx
+       subl    $1b, %edx
 
        /* Load new GDT */
-       leal    gdt(%ebp), %eax
+       leal    gdt(%edx), %eax
        movl    %eax, 2(%eax)
        lgdt    (%eax)
 
@@ -92,13 +92,14 @@ SYM_FUNC_START(startup_32)
        movl    %eax, %ss
 
 /*
- * %ebp contains the address we are loaded at by the boot loader and %ebx
+ * %edx contains the address we are loaded at by the boot loader and %ebx
  * contains the address where we should move the kernel image temporarily
- * for safe in-place decompression.
+ * for safe in-place decompression. %ebp contains the address that the kernel
+ * will be decompressed to.
  */
 
 #ifdef CONFIG_RELOCATABLE
-       movl    %ebp, %ebx
+       movl    %edx, %ebx
        movl    BP_kernel_alignment(%esi), %eax
        decl    %eax
        addl    %eax, %ebx
@@ -110,10 +111,10 @@ SYM_FUNC_START(startup_32)
        movl    $LOAD_PHYSICAL_ADDR, %ebx
 1:
 
+       movl    %ebx, %ebp      // Save the output address for later
        /* Target address to relocate to for decompression */
-       movl    BP_init_size(%esi), %eax
-       subl    $_end, %eax
-       addl    %eax, %ebx
+       addl    BP_init_size(%esi), %ebx
+       subl    $_end, %ebx
 
        /* Set up the stack */
        leal    boot_stack_end(%ebx), %esp
@@ -127,7 +128,7 @@ SYM_FUNC_START(startup_32)
  * where decompression in place becomes safe.
  */
        pushl   %esi
-       leal    (_bss-4)(%ebp), %esi
+       leal    (_bss-4)(%edx), %esi
        leal    (_bss-4)(%ebx), %edi
        movl    $(_bss - startup_32), %ecx
        shrl    $2, %ecx
@@ -196,9 +197,7 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
                                /* push arguments for extract_kernel: */
        pushl   $z_output_len   /* decompressed length, end of relocs */
 
-       leal    _end(%ebx), %eax
-       subl    BP_init_size(%esi), %eax
-       pushl   %eax            /* output address */
+       pushl   %ebp            /* output address */
 
        pushl   $z_input_len    /* input_len */
        leal    input_data(%ebx), %eax