linux-kernel - Re: [Xen-devel] Does __KERNEL

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Message-ID: <CALCETrXUS04--8n7MZ7GvJZi2rU-j=-tPkrjfEMH9+s4SQcg6Q@mail.gmail.com>
Date:	Fri, 8 Apr 2016 15:06:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>, kvm list <kvm@...r.kernel.org>,
	xen-devel <Xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] Does __KERNEL_DS serve a purpose?

On Fri, Apr 8, 2016 at 10:12 AM, Paolo Bonzini <pbonzini@...hat.com> 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.