linuxboot optionrom: do not jump into loaded kernel in a big real mode

PATCH Applied Gleb Natapov 3 participants 3 messages Jan 6, 2013 – Jan 12, 2013

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Gleb Natapov

author

2013-01-06 13:09 UTC #0 lore raw

PATCH v2 1/1 linuxboot optionrom: do not jump into loaded kernel in a big real mode

Big real mode is fully emulated by KVM now, so if control is passed to the loaded kernel while one of the segment registers is in big real mode all the real mode part of the Linux start up is emulated. This slows boot process down. Fix that by resetting ES limit to 0xffff before jumping into the kernel.

The patch also removes unused code segment definition from GDT and changes
ES register to be 16bit in protected mode since CS stays 16bit too and it is CS segment that determines effective operands and addresses length.

Signed-off-by Gleb Natapov <[email protected]>

---
v1->v2
- add patch for binary linuxboot.bin

pc-bios/linuxboot.bin binary +0 -0

index e7c36694f997c3c34f7f4af3c2923bd2ef6094e7..435cac4ebff3fcd83cab4bf74de11f7071ab5aa6 100644
GIT binary patch

pc-bios/optionrom/linuxboot.S +9 -7

index 748c831..afe39a5 100644
@@ -101,18 +101,20 @@ copy_kernel:
 	mov		$1, %eax
 	mov		%eax, %cr0
 
-	/* So we can set ES to a 32-bit segment */
+	/* So we can enlarge ES segment limit */
 	mov		$0x10, %eax
 	mov		%eax, %es
 
-	/* We're now running in 16-bit CS, but 32-bit ES! */
-
 	/* Load kernel and initrd */
 	read_fw_blob_addr32(FW_CFG_KERNEL)
 	read_fw_blob_addr32(FW_CFG_INITRD)
 	read_fw_blob_addr32(FW_CFG_CMDLINE)
 	read_fw_blob_addr32(FW_CFG_SETUP)
 
+	/* Do not leave ES in big real mode  */
+	mov		$0x08, %eax
+	mov		%eax, %es
+
 	/* And now jump into Linux! */
 	mov		$0, %eax
 	mov		%eax, %cr0

@@ -130,10 +132,10 @@ gdt:
 	/* 0x00 */
 .byte	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
 
-	/* 0x08: code segment (base=0, limit=0xfffff, type=32bit code exec/read, DPL=0, 4k) */
-.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x9a, 0xcf, 0x00
+	/* 0x08: data segment (base=0, limit=0xffff, type=16bit data read/write, DPL=0, 4k) */
+.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x00, 0x00
 
-	/* 0x10: data segment (base=0, limit=0xfffff, type=32bit data read/write, DPL=0, 4k) */
-.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0xcf, 0x00
+	/* 0x10: data segment (base=0, limit=0xfffff, type=16bit data read/write, DPL=0, 4k) */
+.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x8f, 0x00
 
 BOOT_ROM_END

--
Gleb.

Alexander Graf

Reviewed-by on v2 1/1

2013-01-06 13:19 UTC #1 lore raw

Quoted 6 lines · Gleb Natapov

Big real mode is fully emulated by KVM now, so if control is passed to the loaded kernel while one of the segment registers is in big real mode all the real mode part of the Linux start up is emulated. This slows boot process down. Fix that by resetting ES limit to 0xffff before jumping into the kernel.

Quoted 7 lines

The patch also removes unused code segment definition from GDT and changes
ES register to be 16bit in protected mode since CS stays 16bit too and it is CS segment that determines effective operands and addresses length.
Signed-off-by Gleb Natapov <[email protected]>

Reviewed-by Alexander Graf <[email protected]>

Alex

Quoted 60 lines

---
v1->v2
- add patch for binary linuxboot.bin

pc-bios/linuxboot.bin binary +0 -0

index e7c36694f997c3c34f7f4af3c2923bd2ef6094e7..435cac4ebff3fcd83cab4bf74de11f7071ab5aa6 100644
GIT binary patch

pc-bios/optionrom/linuxboot.S +5 -3

index 748c831..afe39a5 100644
@@ -101,18 +101,20 @@ copy_kernel:
 	mov		$1, %eax
 	mov		%eax, %cr0
 
-	/* So we can set ES to a 32-bit segment */
+	/* So we can enlarge ES segment limit */
 	mov		$0x10, %eax
 	mov		%eax, %es
 
-	/* We're now running in 16-bit CS, but 32-bit ES! */
-
 	/* Load kernel and initrd */
 	read_fw_blob_addr32(FW_CFG_KERNEL)
 	read_fw_blob_addr32(FW_CFG_INITRD)
 	read_fw_blob_addr32(FW_CFG_CMDLINE)
 	read_fw_blob_addr32(FW_CFG_SETUP)
 
+	/* Do not leave ES in big real mode  */
+	mov		$0x08, %eax
+	mov		%eax, %es
+
 	/* And now jump into Linux! */
 	mov		$0, %eax
 	mov		%eax, %cr0

@@ -130,10 +132,10 @@ gdt:
 	/* 0x00 */

.byte 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

pc-bios/optionrom/linuxboot.S +4 -4

-	/* 0x08: code segment (base=0, limit=0xfffff, type=32bit code exec/read, DPL=0, 4k) */
-.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x9a, 0xcf, 0x00
+	/* 0x08: data segment (base=0, limit=0xffff, type=16bit data read/write, DPL=0, 4k) */
+.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x00, 0x00
 
-	/* 0x10: data segment (base=0, limit=0xfffff, type=32bit data read/write, DPL=0, 4k) */
-.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0xcf, 0x00
+	/* 0x10: data segment (base=0, limit=0xfffff, type=16bit data read/write, DPL=0, 4k) */
+.byte	0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x8f, 0x00

BOOT_ROM_END
--
Gleb.

Blue Swirl

Applied

2013-01-12 16:08 UTC #2 lore raw

In reply to Gleb Natapov

Thanks, applied.

Quoted 74 lines · Gleb Natapov

Signed-off-by Gleb Natapov <[email protected]>

---
v1->v2
- add patch for binary linuxboot.bin

pc-bios/linuxboot.bin binary +0 -0

index e7c36694f997c3c34f7f4af3c2923bd2ef6094e7..435cac4ebff3fcd83cab4bf74de11f7071ab5aa6 100644
GIT binary patch

pc-bios/optionrom/linuxboot.S +9 -8

index 748c831..afe39a5 100644
@@ -101,18 +101,20 @@ copy_kernel:
        mov             $1, %eax
        mov             %eax, %cr0
 
-       /* So we can set ES to a 32-bit segment */
+       /* So we can enlarge ES segment limit */
        mov             $0x10, %eax
        mov             %eax, %es
 
-       /* We're now running in 16-bit CS, but 32-bit ES! */
-
        /* Load kernel and initrd */
        read_fw_blob_addr32(FW_CFG_KERNEL)
        read_fw_blob_addr32(FW_CFG_INITRD)
        read_fw_blob_addr32(FW_CFG_CMDLINE)
        read_fw_blob_addr32(FW_CFG_SETUP)
 
+       /* Do not leave ES in big real mode  */
+       mov             $0x08, %eax
+       mov             %eax, %es
+
        /* And now jump into Linux! */
        mov             $0, %eax
        mov             %eax, %cr0

@@ -130,10 +132,10 @@ gdt:
        /* 0x00 */
 .byte  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
 
-       /* 0x08: code segment (base=0, limit=0xfffff, type=32bit code exec/read, DPL=0, 4k) */
-.byte  0xff, 0xff, 0x00, 0x00, 0x00, 0x9a, 0xcf, 0x00
+       /* 0x08: data segment (base=0, limit=0xffff, type=16bit data read/write, DPL=0, 4k) */
+.byte  0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x00, 0x00
 
-       /* 0x10: data segment (base=0, limit=0xfffff, type=32bit data read/write, DPL=0, 4k) */
-.byte  0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0xcf, 0x00
+       /* 0x10: data segment (base=0, limit=0xfffff, type=16bit data read/write, DPL=0, 4k) */
+.byte  0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x8f, 0x00
 
 BOOT_ROM_END
--
                        Gleb.