[PATCH v3 2/4] x86/boot: Refactor BIOS/PVH start

[Frediano Ziglio <frediano.ziglio@cloud.com>]

The 2 code paths were sharing quite some common code, reuse it instead
of having duplications.
Use %dl register to store boot type before running common code.
Using a 8 bit register reduces code size.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
---
Changes since v1:
- use %dl instead of %ebp to reduce code size;
- fold cli instruction;
- update some comments.
---
 xen/arch/x86/boot/head.S | 117 +++++++++++++++------------------------
 1 file changed, 45 insertions(+), 72 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index fa21024042..80bba6ff21 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -25,6 +25,9 @@
 #define MB2_HT(name)      (MULTIBOOT2_HEADER_TAG_##name)
 #define MB2_TT(name)      (MULTIBOOT2_TAG_TYPE_##name)
 
+#define BOOT_TYPE_BIOS 1
+#define BOOT_TYPE_PVH 2
+
         .macro mb2ht_args arg:req, args:vararg
         .long \arg
         .ifnb \args
@@ -409,17 +412,31 @@ cs32_switch:
 ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .long sym_offs(__pvh_start))
 
 __pvh_start:
-        cld
+        mov     $BOOT_TYPE_PVH, %dl
+        jmp     .Lcommon_bios_pvh
+#endif /* CONFIG_PVH_GUEST */
+
+__start:
+        mov     $BOOT_TYPE_BIOS, %dl
+
+.Lcommon_bios_pvh:
         cli
+        cld
 
         /*
-         * We need one call (i.e. push) to determine the load address.  See
-         * __start for a discussion on how to do this safely using the PVH
-         * info structure.
+         * Multiboot (both 1 and 2) and PVH specify the stack pointer as
+         * undefined.  This is unhelpful for relocatable images, where one
+         * call (i.e. push) is required to calculate the image's load address.
+         *
+         * Durig BIOS boot, there is one area of memory we know about with
+         * reasonable confidence that it isn't overlapped by Xen, and that's
+         * the Multiboot info structure in %ebx.  Use it as a temporary stack.
+         *
+         * During PVH boot use info structure in %ebx.
          */
 
         /* Preserve the field we're about to clobber. */
-        mov     (%ebx), %edx
+        mov     (%ebx), %ecx
         lea     4(%ebx), %esp
 
         /* Calculate the load base address. */
@@ -428,19 +445,12 @@ __pvh_start:
         sub     $sym_offs(1b), %esi
 
         /* Restore the clobbered field. */
-        mov     %edx, (%ebx)
+        mov     %ecx, (%ebx)
 
         /* Set up stack. */
         lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
 
-        call    .Linitialise_bss
-
-        mov     %ebx, sym_esi(pvh_start_info_pa)
-
-        /* Force xen console.  Will revert to user choice in init code. */
-        movb    $-1, sym_esi(opt_console_xen)
-
-        /* Prepare gdt and segments */
+        /* Initialize GDTR and basic data segments. */
         add     %esi, sym_esi(gdt_boot_base)
         lgdt    sym_esi(gdt_boot_descr)
 
@@ -449,62 +459,40 @@ __pvh_start:
         mov     %ecx, %es
         mov     %ecx, %ss
 
-        /* Skip bootloader setup and bios setup, go straight to trampoline */
-        movb    $1, sym_esi(pvh_boot)
-        movb    $1, sym_esi(skip_realmode)
-
-        /* Set trampoline_phys to use mfn 1 to avoid having a mapping at VA 0 */
-        movw    $0x1000, sym_esi(trampoline_phys)
-        mov     (%ebx), %eax /* mov $XEN_HVM_START_MAGIC_VALUE, %eax */
-        jmp     trampoline_setup
-
-#endif /* CONFIG_PVH_GUEST */
+        /* Load null selector to unused segment registers. */
+        xor     %ecx, %ecx
+        mov     %ecx, %fs
+        mov     %ecx, %gs
 
-.Linitialise_bss:
 	/* Initialise the BSS. */
-        mov     %eax, %edx
-
+        mov     %eax, %ebp
         lea     sym_esi(__bss_start), %edi
         lea     sym_esi(__bss_end), %ecx
         sub     %edi, %ecx
         xor     %eax, %eax
         shr     $2, %ecx
         rep stosl
+        mov     %ebp, %eax
 
-        mov     %edx, %eax
-	ret
-
-__start:
-        cld
-        cli
-
-        /*
-         * Multiboot (both 1 and 2) specify the stack pointer as undefined
-         * when entering in BIOS circumstances.  This is unhelpful for
-         * relocatable images, where one call (i.e. push) is required to
-         * calculate the image's load address.
-         *
-         * This early in boot, there is one area of memory we know about with
-         * reasonable confidence that it isn't overlapped by Xen, and that's
-         * the Multiboot info structure in %ebx.  Use it as a temporary stack.
-         */
-
-        /* Preserve the field we're about to clobber. */
-        mov     (%ebx), %edx
-        lea     4(%ebx), %esp
+#ifdef CONFIG_PVH_GUEST
+        cmp     $BOOT_TYPE_PVH, %dl
+        jne     1f
 
-        /* Calculate the load base address. */
-        call    1f
-1:      pop     %esi
-        sub     $sym_offs(1b), %esi
+        mov     %ebx, sym_esi(pvh_start_info_pa)
 
-        /* Restore the clobbered field. */
-        mov     %edx, (%ebx)
+        /* Force xen console.  Will revert to user choice in init code. */
+        movb    $-1, sym_esi(opt_console_xen)
 
-        /* Set up stack. */
-        lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
+        /* Skip bootloader setup and bios setup, go straight to trampoline */
+        movb    $1, sym_esi(pvh_boot)
+        movb    $1, sym_esi(skip_realmode)
 
-        call    .Linitialise_bss
+        /* Set trampoline_phys to use mfn 1 to avoid having a mapping at VA 0 */
+        movl    $PAGE_SIZE, sym_esi(trampoline_phys)
+        mov     (%ebx), %eax /* mov $XEN_HVM_START_MAGIC_VALUE, %eax */
+        jmp     trampoline_setup
+1:
+#endif /* CONFIG_PVH_GUEST */
 
         /* Bootloaders may set multiboot{1,2}.mem_lower to a nonzero value. */
         xor     %edx,%edx
@@ -563,22 +551,7 @@ __start:
 trampoline_bios_setup:
         /*
          * Called on legacy BIOS platforms only.
-         *
-         * Initialize GDTR and basic data segments.
          */
-        add     %esi,sym_esi(gdt_boot_base)
-        lgdt    sym_esi(gdt_boot_descr)
-
-        mov     $BOOT_DS,%ecx
-        mov     %ecx,%ds
-        mov     %ecx,%es
-        mov     %ecx,%ss
-        /* %esp is initialized later. */
-
-        /* Load null descriptor to unused segment registers. */
-        xor     %ecx,%ecx
-        mov     %ecx,%fs
-        mov     %ecx,%gs
 
         /* Set up trampoline segment 64k below EBDA */
         movzwl  0x40e,%ecx          /* EBDA segment */
-- 
2.34.1