Re: [Xen-devel] Does __KERNEL_DS serve a purpose?

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Fri, Apr 8, 2016 at 1:01 AM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 08/04/2016 01:24, Andy Lutomirski wrote:
>> I can't see any reason that we need the __KERNEL_DS segment at all --
>> I think that everything that uses __KERNEL_DS could use __USER_DS
>> instead.  Am I missing anything?  This has been bugging me for a
>> while.
>>
>> I mulled over this a bit when trying to understand the sysret_ss_attrs
>> bug and then forgot about it.
>
> Linux doesn't have a separate __KERNEL_SS.  For the plain data segments,
> the dpl is not interesting.
>
> However, %ss is also loaded with __KERNEL_DS, and %ss.dpl is somewhat
> important.

But %ss can be loaded with 0 on 64-bit kernels.  (I assume that
loading 0 into %ss sets SS.DPL to 0 if done at CPL0, but I'm vague on
this, since it only really matters to hypervisor code AFAIK.)

32-bit kernels need __KERNEL_DS, I think.

--Andy

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