Re: [PATCH kvm-unit-tests] i386: setup segment registers before percpu areas

[Nadav Amit <nadav.amit@gmail.com>]

> On Jun 24, 2020, at 7:14 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> The base of the percpu area is stored in the %gs base, and writing
> to %gs destroys it.  Move setup_segments earlier, before the %gs
> base is written.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> x86/cstart.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/x86/cstart.S b/x86/cstart.S
> index 5ad70b5..77dc34d 100644
> --- a/x86/cstart.S
> +++ b/x86/cstart.S
> @@ -106,6 +106,7 @@ MSR_GS_BASE = 0xc0000101
> .globl start
> start:
>         mov $stacktop, %esp
> +        setup_segments
>         push %ebx
>         call setup_multiboot
>         call setup_libcflat
> @@ -118,7 +119,6 @@ start:
> 
> prepare_32:
>         lgdtl gdt32_descr
> -	setup_segments
> 
> 	mov %cr4, %eax
> 	bts $4, %eax  // pse
> — 
> 2.26.2

As I said in a different thread, this change breaks my setup. It is better
not to make any assumption (or as few as possible) about the GDT content
after boot and load the GDTR before setting up the segments. So I prefer to
load the GDT before the segments. How about this change instead of yours?

-- >8 --

From: Nadav Amit <namit@vmware.com>
Date: Wed, 24 Jun 2020 19:50:36 +0000
Subject: [PATCH] x86: load gdt while loading segments

Signed-off-by: Nadav Amit <namit@vmware.com>
---
 x86/cstart.S | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/x86/cstart.S b/x86/cstart.S
index dd33d4d..1d8b8ac 100644
--- a/x86/cstart.S
+++ b/x86/cstart.S
@@ -95,6 +95,8 @@ MSR_GS_BASE = 0xc0000101
 .endm
 
 .macro setup_segments
+	lgdtl gdt32_descr
+
 	mov $0x10, %ax
 	mov %ax, %ds
 	mov %ax, %es
@@ -106,6 +108,8 @@ MSR_GS_BASE = 0xc0000101
 .globl start
 start:
         mov $stacktop, %esp
+	setup_segments
+
         push %ebx
         call setup_multiboot
         call setup_libcflat
@@ -117,9 +121,6 @@ start:
         jmpl $8, $start32
 
 prepare_32:
-        lgdtl gdt32_descr
-	setup_segments
-
 	mov %cr4, %eax
 	bts $4, %eax  // pse
 	mov %eax, %cr4
--