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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.13 at 18:23, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
> On 03/27/2013 10:17 AM, Stefan Bader wrote:
>>> What does x86info and /proc/cpuinfo show in HVM?
>> 
>> x86info cpuid[7].ebx = 0xbbb and /proc/cpuinfo also shows smep
>> set.
> 
> On all CPUs?
> 
>>> The inbound %cr4 shouldn't matter at all, we try to not rely on
>>> it.
>>> 
>>> If the hypervisor presents SMEP to the guest then the guest is
>>> pretty obviously going to try to use it.
>> 
>> To me it looks like when bootstrapping the APs things are not yet
>> ready to use it. If I did not miss something, the only place that
>> the saved contents of cr4 are used is in startup_32 when the cpus
>> are brought up. And then just stop dead. Would need to read more
>> code but a bit weird why the BP is not affected.
> 
> This feels like a bug in Xen, but I don't know for sure yet.  Either
> which way, it is odd.  That write to cr4 should be entirely legitimate.

And I would guess one that got fixed already.

Stefan, please try 4.2.2-rc1, or (separately)
http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=485f374230d39e153d7b9786e3d0336bd52ee661
(which I think requires the immediately preceding
http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=1e6275a95d3e35a72939b588f422bb761ba82f6b
too).

Jan


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