From 5a08b63cb7fe3e0c03245c55d955e45226e08bc3 Mon Sep 17 00:00:00 2001 From: Michael Brown Date: Thu, 1 May 2014 14:58:24 +0100 Subject: [PATCH] [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 --- src/arch/i386/include/librm.h | 4 ++-- src/arch/i386/transitions/librm.S | 38 +++++++++++++++---------------- 2 files changed, 20 insertions(+), 22 deletions(-) diff --git a/src/arch/i386/include/librm.h b/src/arch/i386/include/librm.h index 4a4e61aa..c8ba72b5 100644 --- a/src/arch/i386/include/librm.h +++ b/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 ) diff --git a/src/arch/i386/transitions/librm.S b/src/arch/i386/transitions/librm.S index 18ceb0d6..0d8110ac 100644 --- a/src/arch/i386/transitions/librm.S +++ b/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 /****************************************************************************