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

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Date:	Thu, 17 Sep 2015 13:39:26 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	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,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops



On 17/09/2015 11:31, Borislav Petkov wrote:
> 
>> > Crashing the bootup on an unknown MSR is bad. Many MSR reads and writes are 
>> > non-critical and returning the 'safe' result is much better than crashing or 
>> > hanging the bootup.
> ... and prepending all MSR accesses with feature/CPUID checks is probably almost
> impossible.

That's not a big deal, that's what *_safe is for.  The problem is that
there are definitely some cases where the *_safe version is not being used.

I agree with Ingo that we should start with a WARN.  For example:

- give the read_msr and write_msr hooks the same prototype as the safe
variants

- make the virt platforms always return "no error" for the unsafe
variants (I understand if your first reaction is "ouch", but this
effectively is already the current behavior)

- change rdmsr/wrmsr/rdmsrl/wrmsrl to WARN if the read_msr and write_msr
hooks return an error

Paolo
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