Re: [PATCH v15 14/23] x86/cpu/intel: Clear SGX_LC capability if not enabled in FEATURE_CONTROL

[Andy Shevchenko]

On Sat, Nov 3, 2018 at 1:17 AM Jarkko Sakkinen
<jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
>
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>
> Like SGX itself, SGX Launch Control must be explicitly enabled via a
> flag in IA32_FEATURE_CONTROL.  Clear the SGX_LC capability if Launch
> Control is not fully enabled (or obviously if SGX itself is disabled).
>
> Note that clearing X86_FEATURE_SGX_LC creates a bit of a conundrum
> regarding the SGXLEPUBKEYHASH MSRs, as it may be desirable to read the
> MSRs even if they are not writable, e.g. to query the configured key,
> but clearing the capability leaves no breadcrum for discerning whether
> or not the MSRs exist.  But, such usage will be rare (KVM is the only
> known case at this time) and not performance critical, so it's not
> unreasonable to require the use of rdmsr_safe().  Clearing the cap bit
> eliminates the need for an additional flag to track whether or not
> Launch Control is truly enabled, which is what we care about the vast
> majority of the time.

> @@ -618,6 +618,8 @@ static void detect_sgx(struct cpuinfo_x86 *c)
>                 setup_clear_cpu_cap(X86_FEATURE_SGX1);
>                 setup_clear_cpu_cap(X86_FEATURE_SGX2);
>         }
> +       if (unsupported || !(fc & FEATURE_CONTROL_SGX_LE_WR))
> +               setup_clear_cpu_cap(X86_FEATURE_SGX_LC);
>  }

A-ha, I see how you use this variable here (though it's still possible
to get rid of it, choose what is better for readability /
maintenance).

-- 
With Best Regards,
Andy Shevchenko