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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 27.03.2013 18:23, H. Peter Anvin 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?

x86info thinks its one core with ht so only one cpuid line for that.
> 
>>> 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.

Could likely be. Unfortunately one where a change in the kernel triggers it. Not
exactly your problem but a pita nonetheless.

-Stefan
> 
>       -hpa
> 
> 
> 
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