x86/boot: Don't overwrite cr4 when enabling PAE

Some EFI firmware makes use of the FPU during boottime services and
clearing X86_CR4_OSFXSR by overwriting %cr4 causes the firmware to
crash.

Add the PAE bit explicitly instead of trashing the existing contents,
leaving the rest of the bits as the firmware set them.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 37c741b0d2ac..4f40cddd025d 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -113,7 +113,8 @@ ENTRY(startup_32)
         lgdt    gdt(%ebp)
 
         /* Enable PAE mode */
-        movl    $(X86_CR4_PAE), %eax
+        movl    %cr4, %eax
+        orl     $X86_CR4_PAE, %eax
         movl    %eax, %cr4
 
  /*