Subject: Re: [Xen-devel] Does __KERNEL_DS serve a purpose?
To: Andy Lutomirski <luto@amacapital.net>,
        Andrew Cooper <andrew.cooper3@citrix.com>
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Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, kvm list <kvm@vger.kernel.org>,
        xen-devel <Xen-devel@lists.xen.org>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5707E681.7040202@redhat.com>
Date: Fri, 8 Apr 2016 19:12:33 +0200
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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.

Paolo

> 32-bit kernels need __KERNEL_DS, I think.