Re: [Xen-devel] Xen HVM regression on certain Intel CPUs

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 27/03/2013 16:45, "Stefan Bader" <stefan.bader@xxxxxxxxxxxxx> wrote:

>>> Seems that I was relying on the wrong source of information when checking
>>> SMEP
>>> support. The cpuid command seems at fail. But /proc/cpuinfo reports it. So
>>> that
>>> at least explains where that comes from... sorry for that.
>> 
>> OK, so if you boot Xen with smep=1 (which disables SMEP, kind of
>> counterintuive flag)
>> that would work fine?
> 
> Rebooting with smep=1 as a hv argument does not fix it. But I would be careful
> since I just quickly did this without checking whether Xen 4.2.1 undestands
> the
> flag already.

Yes, the flag is understood by all Xen 4.2 releases. However it is not
inverted as you believe: it really is smep=0 or smep=off or even no-smep to
disable SMEP. smep=1 will enable SMEP (which is the default anyway).

I also checked how CPUID.SMEP gets set for an HVM guest, and it is very
obviously masked off if SMEP support has been disabled or is unavailable. So
I do not think we can be erroneously passing the CPUID flag to the guest.

 -- Keir



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