linux-kernel - Re: [Xen-devel] [V0 PATCH 1/2] AMD-PVH: set EFER.NX and EFER.SCE for the boot vcpu

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Message-ID: <20140822033228.GA4296@laptop.dumpdata.com>
Date:	Thu, 21 Aug 2014 23:32:28 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Mukesh Rathor <mukesh.rathor@...cle.com>
Cc:	boris.ostrovsky@...cle.com, david.vrabel@...rix.com,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [V0 PATCH 1/2] AMD-PVH: set EFER.NX and EFER.SCE for
 the boot vcpu

On Thu, Aug 21, 2014 at 07:46:56PM -0700, Mukesh Rathor wrote:
> On Thu, 21 Aug 2014 21:39:04 -0400
> Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> 
> > On Wed, Aug 20, 2014 at 07:16:39PM -0700, Mukesh Rathor wrote:
> > > On AMD, NX feature must be enabled in the efer for NX to be honored
> > > in the pte entries, otherwise protection fault. We also set SC for
> > > system calls to be enabled.
> > 
> > How come we don't need to do that for Intel (that is set the NX bit)?
> > Could you include the explanation here please?
> 
> Intel doesn't have EFER.NX bit. The SC bit is being set in xen, but it
> doesn't need to be, and I'm going to submit a patch to undo it.

I understand that it does not have an EFER.NX bit. What I was trying
to figure out is _where_ do we set the NX bit for Intel CPUs? Is
it done in the hypervisor? In the Linux code?

Thank you.
> 
> > 
> > > 
> > > Signed-off-by: Mukesh Rathor <mukesh.rathor@...cle.com>
> > > ---
> > >  arch/x86/xen/enlighten.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> > > index c0cb11f..4af512d 100644
> > > --- a/arch/x86/xen/enlighten.c
> > > +++ b/arch/x86/xen/enlighten.c
> > > @@ -1499,6 +1499,17 @@ void __ref xen_pvh_secondary_vcpu_init(int
> > > cpu) xen_pvh_set_cr_flags(cpu);
> > >  }
> > >  
> > > +/* This is done in secondary_startup_64 for hvm guests. */
> > > +static void __init xen_configure_efer(void)
> > > +{
> > > +	u64 efer;
> > > +
> > > +	rdmsrl(MSR_EFER, efer);
> > > +	efer |= EFER_SCE;
> > > +	efer |= (cpuid_edx(0x80000001) & (1 << 20)) ? EFER_NX : 0;
> > 
> > Ahem? #defines for these magic values please?
> 
> Linux uses these directly all over the code as they are set in stone
> pretty much, and I didn't find any #defines. See cpu/common.c for one of
> the places. Also see secondary_startup_64, and others...
> 
> > Or could you use 'boot_cpu_has'?
> 
> Nop, it's not initialized at this point.
> 
> thanks,
> Mukesh
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