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Subject: Re: [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops
To: Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>
References: <cover.1442445946.git.luto@kernel.org>
 <20150917071920.GA14296@gmail.com> <20150917093134.GC12316@nazgul.tnic>
Cc: Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>, KVM list <kvm@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>
From: Paolo Bonzini <pbonzini@redhat.com>
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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