linux-kernel - Re: [Xen-devel] [V1 PATCH 1/2] PVH: set EFER.NX and EFER.SCE for boot vcpu

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Date:	Thu, 28 Aug 2014 16:59:12 -0700
From:	Mukesh Rathor <mukesh.rathor@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	<boris.ostrovsky@...cle.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [V1 PATCH 1/2] PVH: set EFER.NX and EFER.SCE for
 boot vcpu

On Thu, 28 Aug 2014 15:18:26 +0100
David Vrabel <david.vrabel@...rix.com> wrote:

> On 27/08/14 23:33, Mukesh Rathor wrote:
> > This patch addresses three things for a pvh boot vcpu:
> > 
> >   - NX bug on intel: It was recenlty discovered that NX is not being
> >     honored in PVH on intel since EFER.NX is not being set. The
> > pte.NX bits are ignored if EFER.NX is not set on intel.
> 
> I am unconvinced by this explanation.  The Intel SDM clearly states
> that the XD bit in the page table entries is reserved if EFER.NXE is
> clear, and thus using a entry with XD set and EFER.NXE clear should
> generate a page fault (same as AMD).
> 
> You either need to find out why Intel really worked (perhaps Xen is
> setting EFER.NXE on Intel?) or you need to included an errata (or
> similar) reference.

Nop, verified that again. The vcpu is coming up on efer 0x501,  ie,
LME/LMA/SCE (older xen prior to SCE removal change). The pte entry for
rsp is: 800000003e32b063 that has NX set. No exception is generated upon
push rbp instruction (like on amd).

Could be that Intel docs are incomplete on vmx, I didn't hear back from them 
on the last one I had found. Anyways, we are not addressing an intel errata
here, but fixing our issue of setting the EFER.NX bit.

-Mukesh
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