Re: [PATCH 7/9] x86/hvm: Disable MPX by default

[Jan Beulich <jbeulich@suse.com>]

On 17.06.2020 13:16, Andrew Cooper wrote:
> On 17/06/2020 11:32, Jan Beulich wrote:
>> On 16.06.2020 18:15, Andrew Cooper wrote:
>>> On 16/06/2020 10:33, Jan Beulich wrote:
>>>> On 15.06.2020 16:15, Andrew Cooper wrote:
>>>>> @@ -479,6 +497,18 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid, bool restore,
>>>>>          goto out;
>>>>>      }
>>>>>  
>>>>> +    /*
>>>>> +     * Account for feature which have been disabled by default since Xen 4.13,
>>>>> +     * so migrated-in VM's don't risk seeing features disappearing.
>>>>> +     */
>>>>> +    if ( restore )
>>>>> +    {
>>>>> +        if ( di.hvm )
>>>>> +        {
>>>>> +            p->feat.mpx = test_bit(X86_FEATURE_MPX, host_featureset);
>>>> Why do you derive this from the host featureset instead of the max
>>>> one for the guest type?
>>> Because that is how the logic worked for 4.13.
>>>
>>> Also, because we don't have easy access to the actual guest max
>>> featureset at this point.  I could add two new sysctl subops to
>>> get_featureset, but the reason for not doing so before are still
>>> applicable now.
>>>
>>> There is a theoretical case where host MPX is visible but guest max is
>>> hidden, and that is down to the vmentry controls.  As this doesn't exist
>>> in real hardware, I'm not terribly concerned about it.
>> I'd also see us allow features to be kept for the host, but masked
>> off of the/some guest feature sets, by way of a to-be-introduced
>> command line option.
> 
> What kind of usecase do you have in mind for this?  We've got a load of
> features which are blanket disabled for guests.  I suppose `ler` et al
> ought to have an impact, except for the fact that LBR at that level
> isn't architectural and always expected.

What I was thinking of was the kind of "none of my guests should use
AVX512 - let me disable it globally, rather than individually in
each guest's config" approach. Of course AVX512 is something we use
in Xen only to emulate guest insns, but I think the example still
serves the purpose.

Jan