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

[Andy Lutomirski]

On Fri, Apr 8, 2016 at 10:12 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
>
> On 08/04/2016 18:00, Andy Lutomirski wrote:
>> 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.)
>
> It's even simpler, unless CPL=0 SS cannot be loaded with 0 while in a
> 64-bit code segment (SS can never be loaded with 0 if you're not in a
> 64-bit code segment).
>
> Thus indeed SS=0 implies SS.DPL=0 on 64-bit kernels.

I think we are stuck with __KERNEL_DS: SYSCALL uses it.  Unless we
start fiddling with conforming code segments (ugh), I don't think
there's a valid GDT layout that doesn't have two flat data segments.

Oh well, chalk it up to historical accident.