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arch/x86: (Intel64) optimize and re-order startup assembly sequence

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In some places the code was being overly pedantic; e.g., there is no
need to load our own 32-bit descriptors because the loader's are fine
for our purposes. We can defer loading our own segments until 64-bit.

The sequence is re-ordered to faciliate code sharing between the BSP
and APs when SMP is enabled (all BSP-specific operations occur before
the per-CPU initialization).

Signed-off-by: Charles E. Youse <charles.youse@intel.com>
This commit is contained in:
Charles E. Youse 2019-09-28 17:31:13 -04:00 • committed by Anas Nashif
commit 90bf0da332
1 changed files with 23 additions and 26 deletions
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49
arch/x86/core/intel64/locore.S
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@ -17,23 +17,20 @@
.globl __start
__start:
/*
* kernel execution begins here in 32-bit mode, with flat-mode
* descriptors in all segment registers, interrupts disabled.
* first, let common code do things like detect multiboot info.
*/
#include "../common.S"
/* switch to our own GDT/IDT and stack. */
lgdt gdt48
lidt idt48
jmpl $X86_KERNEL_CS_32, $1f
1: movw $X86_KERNEL_DS_32, %ax
movw %ax, %ds
movw %ax, %ss
movw %ax, %es
movw %ax, %fs
movw %ax, %gs
/*
* clear the BSS. note that we do this in 32-bit mode, so
* the BSS must fit entirely in the first 4GB of RAM.
* N.B.: if multiboot info struct is present, "common.S"
* has left a pointer to it in EBX. do not clobber (yet).
*
* next, clear the BSS. note we're still in 32-bit mode,
* so the BSS must fit entirely in the first 4GB of RAM.
*/
cld
@ -42,9 +39,10 @@ __start:
movl $__bss_num_dwords, %ecx
rep stosl
movl $(_interrupt_stack + CONFIG_ISR_STACK_SIZE), %esp
/* transition to long mode. along the way, we enable SSE. */
/*
* transition to long mode, reload the segment registers,
* and configure per-CPU stuff: GS, task register, stack.
*/
movl %cr4, %eax /* enable PAE and SSE */
orl $(CR4_PAE | CR4_OSFXSR), %eax
@ -63,8 +61,8 @@ __start:
orl $CR0_PG, %eax
movl %eax, %cr0
/* jump into long mode, reload the segment registers (again). */
lgdt gdt48
lidt idt48
jmpl $X86_KERNEL_CS_64, $1f
.code64
1: movl $X86_KERNEL_DS_64, %eax
@ -80,6 +78,12 @@ __start:
movl $(_interrupt_stack + CONFIG_ISR_STACK_SIZE), %esp
/*
* finally, complete environment for the C runtime and go.
*/
cld /* GCC presumes a clear direction flag */
#ifdef CONFIG_INIT_STACKS
movq $0xAAAAAAAAAAAAAAAA, %rax
movq $_interrupt_stack, %rdi
@ -87,11 +91,6 @@ __start:
rep stosq
#endif
/*
* set up SSE in case something uses the floating-point unit during
* early initialization (either directly, or if GCC gets clever)
*/
xorl %edi, %edi
call x86_sse_init
@ -104,8 +103,6 @@ __start:
movl %ebx, %edi /* multiboot pointer (or NULL) */
call z_x86_prep_c /* enter kernel; never returns */
stop: jmp stop
/*
* void x86_sse_init(struct k_thread *thread);
*