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

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Message-ID: <5707E681.7040202@redhat.com>
Date:	Fri, 8 Apr 2016 19:12:33 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	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 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.