[GIT PULL] x86 setup: work around bug in Xen HVM

[GIT PULL] x86 setup: work around bug in Xen HVM

[H. Peter Anvin]

Hi Linus,

Please pull:

  git://git.kernel.org/pub/scm/linux/kernel/git/hpa/linux-2.6-x86setup.git for-linus

Christian Ehrhardt (1):
      [x86 setup] Work around bug in Xen HVM

 arch/i386/boot/pm.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

[Log messages and full diffs follow]

commit 92ea189254b87727d6be407558d9c18fed0937bb
Author: Christian Ehrhardt <lk@c--e.de>
Date:   Mon Sep 3 20:32:38 2007 +0200

    [x86 setup] Work around bug in Xen HVM
    
    Apparently XEN does not keep the contents of the 48-bit gdt_48 data
    structure that is passed to lgdt in the XEN machine state. Instead it
    appears to save the _address_ of the 48-bit descriptor
    somewhere. Unfortunately this data happens to reside on the stack and
    is probably no longer availiable at the time of the actual protected
    mode jump.
    
    This is Xen bug but given that there is a one-line patch to work
    around this problem, the linux kernel should probably do this.  My fix
    is to make the gdt_48 description in setup_gdt static (in setup_idt
    this is already the case). This allows the kernel to boot under
    Xen HVM again.
    
    Signed-off-by: Christian Ehrhardt <lk@c--e.de>
    Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/arch/i386/boot/pm.c b/arch/i386/boot/pm.c
index 6be9ca8..f7958f1 100644
--- a/arch/i386/boot/pm.c
+++ b/arch/i386/boot/pm.c
@@ -122,7 +122,7 @@ static void setup_gdt(void)
 		/* DS: data, read/write, 4 GB, base 0 */
 		[GDT_ENTRY_BOOT_DS] = GDT_ENTRY(0xc093, 0, 0xfffff),
 	};
-	struct gdt_ptr gdt;
+	static struct gdt_ptr gdt;
 
 	gdt.len = sizeof(boot_gdt)-1;
 	gdt.ptr = (u32)&boot_gdt + (ds() << 4);

Re: [GIT PULL] x86 setup: work around bug in Xen HVM

[Christoph Hellwig]

On Tue, Sep 04, 2007 at 09:55:45AM -0700, H. Peter Anvin wrote:
>     
>     Apparently XEN does not keep the contents of the 48-bit gdt_48 data
>     structure that is passed to lgdt in the XEN machine state. Instead it
>     appears to save the _address_ of the 48-bit descriptor
>     somewhere. Unfortunately this data happens to reside on the stack and
>     is probably no longer availiable at the time of the actual protected
>     mode jump.
>     
>     This is Xen bug but given that there is a one-line patch to work
>     around this problem, the linux kernel should probably do this.  My fix
>     is to make the gdt_48 description in setup_gdt static (in setup_idt
>     this is already the case). This allows the kernel to boot under
>     Xen HVM again.

> -	struct gdt_ptr gdt;
> +	static struct gdt_ptr gdt;

It might make sense to add your above commit message to the code as a comment.

Re: [GIT PULL] x86 setup: work around bug in Xen HVM

[H. Peter Anvin]

Christoph Hellwig wrote:
> On Tue, Sep 04, 2007 at 09:55:45AM -0700, H. Peter Anvin wrote:
>>     
>>     Apparently XEN does not keep the contents of the 48-bit gdt_48 data
>>     structure that is passed to lgdt in the XEN machine state. Instead it
>>     appears to save the _address_ of the 48-bit descriptor
>>     somewhere. Unfortunately this data happens to reside on the stack and
>>     is probably no longer availiable at the time of the actual protected
>>     mode jump.
>>     
>>     This is Xen bug but given that there is a one-line patch to work
>>     around this problem, the linux kernel should probably do this.  My fix
>>     is to make the gdt_48 description in setup_gdt static (in setup_idt
>>     this is already the case). This allows the kernel to boot under
>>     Xen HVM again.
> 
>> -	struct gdt_ptr gdt;
>> +	static struct gdt_ptr gdt;
> 
> It might make sense to add your above commit message to the code as a comment.

Good point; I have amended the commit with a brief comment:

diff --git a/arch/i386/boot/pm.c b/arch/i386/boot/pm.c
index 6be9ca8..09fb342 100644
--- a/arch/i386/boot/pm.c
+++ b/arch/i386/boot/pm.c
@@ -122,7 +122,11 @@ static void setup_gdt(void)
                 /* DS: data, read/write, 4 GB, base 0 */
                 [GDT_ENTRY_BOOT_DS] = GDT_ENTRY(0xc093, 0, 0xfffff),
         };
-       struct gdt_ptr gdt;
+       /* Xen HVM incorrectly stores a pointer to the gdt_ptr, instead
+          of the gdt_ptr contents.  Thus, make it static so it will
+          stay in memory, at least long enough that we switch to the
+          proper kernel GDT. */
+       static struct gdt_ptr gdt;

         gdt.len = sizeof(boot_gdt)-1;
         gdt.ptr = (u32)&boot_gdt + (ds() << 4);

	-hpa