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