[librm] Speed up protected-to-real mode transition under KVM

On an Intel CPU supporting VMX, KVM will emulate instructions while
the CPU state remains "invalid".  In real mode, the CPU state is
defined to be "invalid" if any segment register has a base which is
not equal to (sreg<<4) or a limit which is not equal to 64kB.

We don't actually use the base stored in the REAL_DS descriptor for
any significant purpose.  Change the base stored in this descriptor to
be equal to (REAL_DS<<4).  A segment register loaded with REAL_DS is
then automatically valid in both real and protected modes.  This
allows KVM to stop emulating instructions much sooner.

The only use of REAL_DS for memory accesses currently occurs in the
indirect ljmp within prot_to_real.  Change this to a direct ljmp,
storing rm_cs in .text16 as part of the ljmp instruction.  This
removes the only memory access via REAL_DS (thereby allowing for the
above descriptor base address hack), and also simplifies the ljmp
instruction (which will still have to be emulated).

Load the real-mode interrupt descriptor table register before
switching to real mode, since this avoids triggering an EXCEPTION_NMI
and corresponding VM exit.

This reduces the time taken by prot_to_real under KVM by around 65%.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/include/librm.h

@@ -165,8 +165,8 @@ extern char *text16;
 /* Variables in librm.S, present in the normal data segment */
 extern uint16_t rm_sp;
 extern uint16_t rm_ss;
-extern uint16_t __data16 ( rm_cs );
-#define rm_cs __use_data16 ( rm_cs )
+extern uint16_t __text16 ( rm_cs );
+#define rm_cs __use_text16 ( rm_cs )
 extern uint16_t __text16 ( rm_ds );
 #define rm_ds __use_text16 ( rm_ds )
 

src/arch/i386/transitions/librm.S

@@ -72,7 +72,7 @@ real_cs: 	/* 16 bit real mode code segment */
 
 	.org	gdt + REAL_DS	
 real_ds:	/* 16 bit real mode data segment */
-	.word	0xffff, 0
+	.word	0xffff, ( REAL_DS << 4 )
 	.byte	0, 0x93, 0x00, 0
 
 gdt_end:
@@ -111,20 +111,18 @@ init_librm:
 	/* Store rm_cs and text16, set up real_cs segment */
 	xorl	%eax, %eax
 	movw	%cs, %ax
-	movw	%ax, rm_cs
+	movw	%ax, %cs:rm_cs
 	shll	$4, %eax
 	movw	$real_cs, %bx
 	call	set_seg_base
 	addr32 leal	(%eax, %edi), %ebx
 	movl	%ebx, rm_text16
 
-	/* Store rm_ds and data16, set up real_ds segment */
+	/* Store rm_ds and data16 */
 	xorl	%eax, %eax
 	movw	%ds, %ax
 	movw	%ax, %cs:rm_ds
 	shll	$4, %eax
-	movw	$real_ds, %bx
-	call	set_seg_base
 	addr32 leal	(%eax, %edi), %ebx
 	movl	%ebx, rm_data16
 
@@ -241,7 +239,7 @@ r2p_pmode:
 	ret
 
 	/* Default real-mode interrupt descriptor table */
-	.section ".data16", "aw", @progbits
+	.section ".data", "aw", @progbits
 rm_idtr:
 	.word 0xffff /* limit */
 	.long 0 /* base */
@@ -287,6 +285,9 @@ prot_to_real:
 	/* Record protected-mode %esp (after removal of data) */
 	movl	%esi, pm_esp
 
+	/* Reset IDTR to the real-mode defaults */
+	lidt	rm_idtr
+
 	/* Load real-mode segment limits */
 	movw	$REAL_DS, %ax
 	movw	%ax, %ds
@@ -302,9 +303,9 @@ p2r_rmode:
 	movl	%cr0, %eax
 	andb	$0!CR0_PE, %al
 	movl	%eax, %cr0
-	ljmp	*p2r_jump_vector
-p2r_jump_target:
-
+p2r_ljmp_rm_cs:
+	ljmp	$0, $1f
+1:
 	/* Set up real-mode data segments and stack pointer */
 	movw	%cs:rm_ds, %ax
 	movw	%ax, %ds
@@ -314,26 +315,23 @@ p2r_jump_target:
 	movw	%bp, %ss
 	movl	%edx, %esp
 
-	/* Reset IDTR to the real-mode defaults */
-	data32 lidt rm_idtr
-
 	/* Return to real-mode address */
 	data32 ret
 
 
 	/* Real-mode code and data segments.  Assigned by the call to
 	 * init_librm.  rm_cs doubles as the segment part of the jump
-	 * vector used by prot_to_real.  rm_ds is located in .text16
-	 * rather than .data16 because code needs to be able to locate
-	 * the data segment.
+	 * instruction used by prot_to_real.  Both are located in
+	 * .text16 rather than .data16: rm_cs since it forms part of
+	 * the jump instruction within the code segment, and rm_ds
+	 * since real-mode code needs to be able to locate the data
+	 * segment with no other reference available.
 	 */
-	.section ".data16", "aw", @progbits
-p2r_jump_vector:
-	.word	p2r_jump_target
 	.globl rm_cs
-rm_cs:	.word 0
-	.globl rm_ds
+	.equ	rm_cs, ( p2r_ljmp_rm_cs + 3 )
+
 	.section ".text16.data", "aw", @progbits
+	.globl rm_ds
 rm_ds:	.word 0
 
 /****************************************************************************