linux-kernel - Re: [Xen-devel] [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS environments

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Message-Id: <553628240200007800074272@mail.emea.novell.com>
Date:	Tue, 21 Apr 2015 09:36:20 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andrew Cooper" <andrew.cooper3@...rix.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>, <x86@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>, <lguest@...ts.ozlabs.org>,
	"Xen-devel" <xen-devel@...ts.xen.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	<linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] [RFC] x86/cpu: Fix SMAP check in PVOPS
 environments

>>> On 20.04.15 at 19:09, <andrew.cooper3@...rix.com> wrote:
> A different approach, given the dual nature of the AC flag now is to gate
> setup_smap() on a kernel rpl of 0.  SMAP necessarily can't be used in a
> paravirtual situation where the kernel runs in cpl > 0.

"Can't" isn't true here - for 64-bit PV Xen guests, which already
toggle between two page table variants for kernel and user mode,
it would be possible (but perhaps expensive) to mimic the needed
behavior by introducing a 3rd set of page tables, containing only
the kernel mappings. You may recall that I had even posted an
RFC patch to tat effect about a year ago.

Jan

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