Re: [Xen-devel] [PATCH] x86: introduce and use setup_force_cpu_cap()

[Sarah Newman <srn@xxxxxxxxx>]

On 09/05/2017 06:22 AM, Jan Beulich wrote:
> For XEN_SMEP and XEN_SMAP to not be cleared while bringing up APs we'd
> need to clone the respective hack used for CPUID_FAULTING. Introduce an
> inverse of setup_clear_cpu_cap() instead, but let clearing of features
> overrule forced setting of them.
> 
> XEN_SMAP being wrong post-boot is a problem specifically for live
> patching, as a live patch may need alternative instruction patching
> keyed off of that feature flag.
> 
> Reported-by: Sarah Newman <security@xxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reported-by/Tested-by: Sarah Newman <srn@xxxxxxxxx>

Some questions:

It looks like setup_clear_cpu_cap has a search for dependent capabilities that 
also must be cleared. Does the same need to happen for
setup_force_cpu_cap even if it doesn't matter for the current cpu features?

Does it make sense to add a comment where forced_caps is declared that 
cleared_caps overrides forced_caps instead of just in the commit message?

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