linux-kernel - Re: [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops

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Date:	Thu, 17 Sep 2015 10:58:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
	KVM list <kvm@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	xen-devel <Xen-devel@...ts.xen.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops

On Wed, Sep 16, 2015 at 04:33:11PM -0700, Andy Lutomirski wrote:
> Setting CONFIG_PARAVIRT=y has an unintended side effect: it silently
> turns all rdmsr and wrmsr operations into the safe variants without
> any checks that the operations actually succeed.
> 
> This is IMO awful: it papers over bugs.  In particular, KVM gueests
> might be unwittingly depending on this behavior because
> CONFIG_KVM_GUEST currently depends on CONFIG_PARAVIRT.  I'm not
> aware of any such problems, but applying this series would be a good
> way to shake them out.
> 
> Fix it so that the MSR operations work the same on CONFIG_PARAVIRT=n
> and CONFIG_PARAVIRT=y as long as Xen isn't being used.  The Xen
> maintainers are welcome to make a similar change on top of this.
> 
> Since there's plenty of time before the next merge window, I think
> we should apply and fix anything that breaks.
> 
> Doing this is probably a prerequisite to sanely decoupling
> CONFIG_KVM_GUEST and CONFIG_PARAVIRT, which would probably make
> Arjan and the rest of the Clear Containers people happy :)

So I actually like this, although by Ingo's argument, its a tad risky.

But the far greater problem I have with the whole virt thing is that
you cannot use rdmsr_safe() to probe if an MSR exists at all because, as
you told me, these virt thingies return 0 for all 'unknown' MSRs instead
of faulting.
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