From: Cao jin <caoj.fnst@cn.fujitsu.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>
Cc: <kirill.shutemov@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>
Subject: question about head_64.S
Date: Tue, 15 Jan 2019 19:45:10 +0800 [thread overview]
Message-ID: <6aebbf86-2ba1-c517-dc47-054279daec49@cn.fujitsu.com> (raw)
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
next reply other threads:[~2019-01-15 11:44 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-15 11:45 Cao jin [this message]
2019-01-15 15:55 ` question about head_64.S Thomas Gleixner
2019-01-16 9:44 ` Cao jin
2019-01-22 7:31 ` Cao jin
2019-01-22 13:08 ` Kirill A. Shutemov
2019-01-23 4:01 ` Cao jin
2019-01-23 10:03 ` Kirill A. Shutemov
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