x86, boot: set up the decompression stack as early as possible

Set up the decompression stack as soon as we know where it needs to
go.  That way we have a full-service stack as soon as possible, rather
than relying on the BP_scratch field.

Note that the stack does need to be empty during bss zeroing (or
else the stack needs to be moved out of the bss segment, which is also
an option.)

[ Impact: cleanup, minor paranoia ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

2 files changed, 12 insertions, 14 deletions

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 59425e157df3..d7245cf80261 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -88,6 +88,9 @@ ENTRY(startup_32)
         addl    $4095, %ebx
         andl    $~4095, %ebx
 
+        /* Set up the stack */
+        leal    boot_stack_end(%ebx), %esp
+
 /*
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
@@ -122,7 +125,7 @@ ENDPROC(startup_32)
 relocated:
 
 /*
- * Clear BSS
+ * Clear BSS (stack is currently empty)
  */
         xorl    %eax, %eax
         leal    _bss(%ebx), %edi
@@ -132,11 +135,6 @@ relocated:
         rep     stosb
 
 /*
- * Setup the stack for the decompressor
- */
-        leal    boot_stack_end(%ebx), %esp
-
-/*
  * Do the decompression, and jump to the new kernel..
  */
         movl    output_len(%ebx), %eax
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 5bc9052615b6..a0b18426069a 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -249,6 +249,13 @@ ENTRY(startup_64)
         addq    $(32768 + 18 + 4095), %rbx
         andq    $~4095, %rbx
 
+        /* Set up the stack */
+        leaq    boot_stack_end(%rbx), %rsp
+
+        /* Zero EFLAGS */
+        pushq   $0
+        popfq
+
 /*
  * Copy the compressed kernel to the end of our buffer
  * where decompression in place becomes safe.
@@ -273,7 +280,7 @@ ENTRY(startup_64)
 relocated:
 
 /*
- * Clear BSS
+ * Clear BSS (stack is currently empty)
  */
         xorq    %rax, %rax
         leaq    _bss(%rbx), %rdi
@@ -282,13 +289,6 @@ relocated:
         cld
         rep     stosb
 
-        /* Setup the stack */
-        leaq    boot_stack_end(%rip), %rsp
-
-        /* zero EFLAGS after setting rsp */
-        pushq   $0
-        popfq
-
 /*
  * Do the decompression, and jump to the new kernel..
  */