Messages in this thread Patch in this message | | | From | Cao jin <> | Subject | question about head_64.S | Date | Tue, 15 Jan 2019 19:45:10 +0800 |
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Hi, I have been digging into this file for a while, and I still have 2 questions unclear, hope to get your help.
1. At the entry of startup_64, we set all the data segment registers to 0, according to commit 08da5a2ca("x86_64: Early segment setup for VT"), it is said to accelerate the decompression under VT. I don't know Intel VT, but I did test under physical machine and virtual machine(with KVM, and intel VT enabled in BIOS) with following patch:
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 58f6a467f1fa..595f3c300173 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -260,12 +260,12 @@ ENTRY(startup_64) */
/* Setup data segments. */ - xorl %eax, %eax - movl %eax, %ds - movl %eax, %es - movl %eax, %ss - movl %eax, %fs - movl %eax, %gs +// xorl %eax, %eax +// movl %eax, %ds +// movl %eax, %es +// movl %eax, %ss +// movl %eax, %fs +// movl %eax, %gs
I don't see any obvious booting time difference, is there anything I missed? Also, I don't find explicit document saying we should zero these registers under VT.
2. Why gdt64 has following definition?:
gdt64: .word gdt_end - gdt .long 0 .word 0 .quad 0 obviously, gdt64 stores the GDTR content under x86_64, which is 10 bytes long, so why not just:
gdt64: .word gdt_end - gdt .quad 0
With above modification, it can boot. -- Sincerely, Cao jin
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