[vfio-users] [FEEDBACK NEEDED] Rewriting the Arch wiki article
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.
> vfio-users mailing list
> vfio-users at redhat.com
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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