[vfio-users] [FEEDBACK NEEDED] Rewriting the Arch wiki article

Nicolas Roy-Renaud nicolas.roy-renaud.1 at ens.etsmtl.ca
Mon Apr 18 19:49:33 UTC 2016

On 2016-04-15 18:51, Hristo Iliev wrote:
> On Tue, 12 Apr 2016 14:50:55 -0600 Will Marler <will at wmarler.com> wrote:
>> "options kvm ignore_msrs=1"
>> On the wiki page as it stands now there is a warning, "Silently ignoring
>> unknown MSR accesses could potentially break other software within the VM
>> or other VMs." I'm not clear on all of the ramifications, though it came up
>> on the mailing list before. I basically decided "not important enough for
>> me to do it" and forgot all the details. But I wouldn't remove this warning
>> or suggest this as a performance gain unless the risks have been eliminated
>> (or are only applicable to specific edge cases or something).
> As I've probably said before, I wrote this section of the wiki. ignore_msrs=1
> stops KVM from injecting #GPs when unknown MSRs are accessed and simply makes
> the reads return 0 and turns the writes into no-ops. While it stops certain
> software from crashing as a result of the unexpected protection faults, it
> could result in the same software silently making wrong decisions based on the
> zero return value and it might break legitimate software that is already
> properly handling the faults. It basically replaces a well-defined exception
> with a junk value.
> In my case, I'm doing CPUID passthrough with my Haswell-E CPU and the unknown
> MSRs are always Intel power and frequency management registers, which are
> supposed to exist on the real Haswell chip. Ignoring them workes for the
> NVIDIA tool, but I would not make a general case out of a specific one,
> therefore the warning.
> Regards,
> Hristo
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I've changed it to make it clearer that it doesn't simply apply to 
Geforce Experience, as others have suggested, and reworded the warning 
to mention that it should /normally/ be safe to do this.

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