x86/boot: Remove KEEP_SEGMENTS support

Commit a24e7851 ("i386: paravirt boot sequence") added this flag for
use by paravirtualized environments such as Xen. However, Xen never made
use of this flag [1], and it was only ever used by lguest [2].

Commit ecda85e7 ("x86/lguest: Remove lguest support") removed
lguest, so KEEP_SEGMENTS has lost its last user.

[1] https://lore.kernel.org/lkml/4D4B097C.5050405@goop.org
[2] https://www.mail-archive.com/lguest@lists.ozlabs.org/msg00469.htmlSigned-off-by: NArvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-2-nivedita@alum.mit.eduSigned-off-by: NArd Biesheuvel <ardb@kernel.org>

Documentation/x86/boot.rst

@@ -490,15 +490,11 @@ Protocol:	2.00+
 		kernel) to not write early messages that require
 		accessing the display hardware directly.
 
-  Bit 6 (write): KEEP_SEGMENTS
+  Bit 6 (obsolete): KEEP_SEGMENTS
 
 	Protocol: 2.07+
 
-	- If 0, reload the segment registers in the 32bit entry point.
-	- If 1, do not reload the segment registers in the 32bit entry point.
-
-		Assume that %cs %ds %ss %es are all set to flat segments with
-		a base of 0 (or the equivalent for their environment).
+        - This flag is obsolete.
 
   Bit 7 (write): CAN_USE_HEAP
 

arch/x86/boot/compressed/head_32.S

@@ -63,13 +63,6 @@
 	__HEAD
 SYM_FUNC_START(startup_32)
 	cld
-	/*
-	 * Test KEEP_SEGMENTS flag to see if the bootloader is asking
-	 * us to not reload segments
-	 */
-	testb	$KEEP_SEGMENTS, BP_loadflags(%esi)
-	jnz	1f
-
 	cli
 	movl	$__BOOT_DS, %eax
 	movl	%eax, %ds
@@ -77,7 +70,6 @@ SYM_FUNC_START(startup_32)
 	movl	%eax, %fs
 	movl	%eax, %gs
 	movl	%eax, %ss
-1:
 
 /*
  * Calculate the delta between where we were compiled to run

arch/x86/boot/compressed/head_64.S

@@ -53,19 +53,11 @@ SYM_FUNC_START(startup_32)
 	 * all need to be under the 4G limit.
 	 */
 	cld
-	/*
-	 * Test KEEP_SEGMENTS flag to see if the bootloader is asking
-	 * us to not reload segments
-	 */
-	testb $KEEP_SEGMENTS, BP_loadflags(%esi)
-	jnz 1f
-
 	cli
 	movl	$(__BOOT_DS), %eax
 	movl	%eax, %ds
 	movl	%eax, %es
 	movl	%eax, %ss
-1:
 
 /*
  * Calculate the delta between where we were compiled to run

arch/x86/kernel/head_32.S

@@ -67,11 +67,6 @@ __HEAD
 SYM_CODE_START(startup_32)
 	movl pa(initial_stack),%ecx
 	
-	/* test KEEP_SEGMENTS flag to see if the bootloader is asking
-		us to not reload segments */
-	testb $KEEP_SEGMENTS, BP_loadflags(%esi)
-	jnz 2f
-
 /*
  * Set segments to known values.
  */
@@ -82,7 +77,6 @@ SYM_CODE_START(startup_32)
 	movl %eax,%fs
 	movl %eax,%gs
 	movl %eax,%ss
-2:
 	leal -__PAGE_OFFSET(%ecx),%esp
 
 /*