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Date:	Mon, 14 Jul 2014 10:11:13 -0700
From:	Andy Lutomirski <>
To:	"H. Peter Anvin" <>
Cc:	Konrad Rzeszutek Wilk <>,
	"" <>
Subject: Re: Is espfix64's double-fault thing OK on Xen?

On Mon, Jul 14, 2014 at 10:04 AM, H. Peter Anvin <> wrote:
> On 07/09/2014 04:17 PM, Andy Lutomirski wrote:
>> This part in __do_double_fault looks fishy:
>>     cmpl $__KERNEL_CS,CS(%rdi)
>>     jne do_double_fault
>> Shouldn't that be:
>>     test $3,CS(%rdi)
>>     jnz do_double_fault
> No, it should be fine.  The *only* case where we need to do the espfix
> magic is when we are on __KERNEL_CS.

IIRC Xen has a somewhat different GDT, and at least the userspace CS
in IA32_STAR disagrees with normal Linux.  If the kernel CS is also
strange, then there will be an extra possible CS value here.

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