linux-kernel - Re: [tip:x86/cpu] x86, AMD: Enable WC+ memory type on family 10 processors

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Date:	Wed, 13 Feb 2013 12:16:27 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	boris.ostrovsky@....com, tglx@...utronix.de, hpa@...ux.intel.com,
	linux-tip-commits@...r.kernel.org,
	Sasha Levin <sasha.levin@...cle.com>,
	Asias He <asias@...hat.com>
Subject: Re: [tip:x86/cpu] x86, AMD: Enable WC+ memory type on family 10
 processors

On Wed, Feb 13, 2013 at 10:06:25AM +0200, Pekka Enberg wrote:
> On Wed, Feb 13, 2013 at 2:21 AM, H. Peter Anvin <hpa@...or.com> wrote:
> >>> +                       rdmsrl(MSR_AMD64_BU_CFG2, value);
> >>> +                       value &= ~(1ULL << 24);
> >>> +                       wrmsrl(MSR_AMD64_BU_CFG2, value);
> >>> +               }
> >>>         }
> >>>
> >>>         rdmsr_safe(MSR_AMD64_PATCH_LEVEL, &c->microcode, &dummy);
> >>
> >>
> >> However, the more serious issue is that that same kernel #GPs when
> >> booted in kvm. It seems it cannot stomach that specific MSR, see the
> >> second "<-- trapping instruction" below and that BU_CFG2 MSR landing in
> >> %ecx in the line before that.
> >>
> >> Oh, and this happens only with the kvm executable (/usr/bin/kvm) in
> >> debian testing. If I use qemu from git, it passes over init_amd just
> >> fine.
> >>
> >> Hmmm..
> >
> > It #GPs on an MSR, which tends to be a bug in the VMM; RDMSR/WRMSR generally
> > kick out to the VMM.  There isn't a huge lot of work we can do about that...
> >
> > I think Qemu defaults to ignoring unknown-to-it MSRs whereas maybe kvmtool
> > croaks?  Pekka?
> 
> I think we should also be fine. Cyrill?

Never was #gp on unknown msr register access, so I think
we're safe. But maybe Borislav may give lkvm a shot just
to be sure?
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