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[librm] Speed up protected-mode calls under KVM
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When making a call from real mode to protected mode, we save and
restore the global and interrupt descriptor table registers.  The
restore currently takes place after returning to real mode, which
generates two EXCEPTION_NMIs and corresponding VM exits when running
under KVM on an Intel CPU.

Avoid the VM exits by restoring the descriptor table registers inside
prot_to_real, while still running in protected mode.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
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@mcb30
mcb30 committed May 2, 2014
1 parent c64747d commit bcfaf11
Showing 1 changed file with 35 additions and 19 deletions.
54 changes: 35 additions & 19 deletions src/arch/i386/transitions/librm.S
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Expand Up @@ -253,12 +253,6 @@ r2p_pmode:
/* Return to virtual address */
ret

/* Default real-mode interrupt descriptor table */
.section ".data", "aw", @progbits
rm_idtr:
.word 0xffff /* limit */
.long 0 /* base */

/****************************************************************************
* prot_to_real (protected-mode near call, 32-bit real-mode return address)
*
Expand All @@ -275,12 +269,22 @@ rm_idtr:
*
* Parameters:
* %ecx : number of bytes to move from PM stack to RM stack
* %esi : real-mode global and interrupt descriptor table registers
*
****************************************************************************
*/
.section ".text", "ax", @progbits
.code32
prot_to_real:
/* Copy real-mode global descriptor table register to RM code segment */
movl text16, %edi
leal rm_gdtr(%edi), %edi
movsw
movsl

/* Load real-mode interrupt descriptor table register */
lidt (%esi)

/* Add return address to data to be moved to RM stack */
addl $4, %ecx

Expand All @@ -300,9 +304,6 @@ 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
Expand All @@ -314,6 +315,8 @@ prot_to_real:
.section ".text16", "ax", @progbits
.code16
p2r_rmode:
/* Load real-mode GDT */
data32 lgdt %cs:rm_gdtr
/* Switch to real mode */
movl %cr0, %eax
andb $0!CR0_PE, %al
Expand Down Expand Up @@ -349,6 +352,12 @@ p2r_ljmp_rm_cs:
.globl rm_ds
rm_ds: .word 0

/* Real-mode global and interrupt descriptor table registers */
.section ".text16.data", "aw", @progbits
rm_gdtr:
.word 0 /* Limit */
.long 0 /* Base */

/****************************************************************************
* prot_call (real-mode far call, 16-bit real-mode far return address)
*
Expand Down Expand Up @@ -384,8 +393,8 @@ rm_ds: .word 0
*/

#define PC_OFFSET_GDT ( 0 )
#define PC_OFFSET_IDT ( PC_OFFSET_GDT + 8 /* pad to 8 to keep alignment */ )
#define PC_OFFSET_IX86 ( PC_OFFSET_IDT + 8 /* pad to 8 to keep alignment */ )
#define PC_OFFSET_IDT ( PC_OFFSET_GDT + 6 )
#define PC_OFFSET_IX86 ( PC_OFFSET_IDT + 6 )
#define PC_OFFSET_RETADDR ( PC_OFFSET_IX86 + SIZEOF_I386_ALL_REGS )
#define PC_OFFSET_FUNCTION ( PC_OFFSET_RETADDR + 4 )
#define PC_OFFSET_END ( PC_OFFSET_FUNCTION + 4 )
Expand All @@ -403,10 +412,10 @@ prot_call:
pushw %ds
pushw %ss
pushw %cs
subw $16, %sp
subw $PC_OFFSET_IX86, %sp
movw %sp, %bp
sidt 8(%bp)
sgdt (%bp)
sidt PC_OFFSET_IDT(%bp)
sgdt PC_OFFSET_GDT(%bp)

/* For sanity's sake, clear the direction flag as soon as possible */
cld
Expand All @@ -426,16 +435,14 @@ pc_pmode:

/* Switch to real mode and move register dump back to RM stack */
movl $PC_OFFSET_END, %ecx
movl %esp, %esi
pushl $pc_rmode
jmp prot_to_real
.section ".text16", "ax", @progbits
.code16
pc_rmode:
/* Reload GDT and IDT, restore registers and flags and return */
movw %sp, %bp
data32 lgdt (%bp)
data32 lidt 8(%bp)
addw $20, %sp /* also skip %cs and %ss */
/* Restore registers and flags and return */
addw $( PC_OFFSET_IX86 + 4 /* also skip %cs and %ss */ ), %sp
popw %ds
popw %es
popw %fs
Expand Down Expand Up @@ -489,6 +496,7 @@ real_call:
/* Switch to real mode and move register dump to RM stack */
movl $( RC_OFFSET_RETADDR + 4 /* function pointer copy */ ), %ecx
pushl $rc_rmode
movl $rm_default_gdtr_idtr, %esi
jmp prot_to_real
.section ".text16", "ax", @progbits
.code16
Expand Down Expand Up @@ -520,6 +528,14 @@ rc_pmode:
.section ".data16", "aw", @progbits
rc_function: .word 0, 0

/* Default real-mode global and interrupt descriptor table registers */
.section ".data", "aw", @progbits
rm_default_gdtr_idtr:
.word 0 /* Global descriptor table limit */
.long 0 /* Global descriptor table base */
.word 0x03ff /* Interrupt descriptor table limit */
.long 0 /* Interrupt descriptor table base */

/****************************************************************************
* flatten_real_mode (real-mode near call)
*
Expand Down

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