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

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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>
This commit is contained in:
Michael Brown 2014-05-01 14:58:24 +01:00
parent 03e76c34d8
commit 5a08b63cb7
2 changed files with 20 additions and 22 deletions
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4
src/arch/i386/include/librm.h
View File

@ -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 )

38
src/arch/i386/transitions/librm.S
View File

@ -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
/****************************************************************************