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Subjectquestion about head_64.S
Date
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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