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

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Date:	Wed, 05 Sep 2007 02:39:54 +0100
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Hellwig <hch@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christian Ehrhardt <lk@...e.de>
Subject: Re: [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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