Re: [PATCH 2/4] x86: introduce disabled-features

[Borislav Petkov]

On Wed, Jul 02, 2014 at 09:29:28AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> I believe the REQUIRED_MASK aproach was taken so that it was
> easier to consult in assembly (arch/x86/kernel/verify_cpu.S).
> DISABLED_MASK does not have the same restriction, but I
> implemented it the same way for consistency.
> 
> We have a REQUIRED_MASK... which does two things:
> 1. Keeps a list of cpuid bits to check in very early boot and
>    refuse to boot if those are not present.
> 2. Consulted during cpu_has() checks, which allows us to
>    optimize out things at compile-time.  In other words, if we
>    *KNOW* we will not boot with the feature off, then we can
>    safely assume that it will be present forever.
> 
> But, we don't have a similar mechanism for CPU features which
> may be present but that we know we will not use.  We simply
> use our existing mechanisms to repeatedly check the status of
> the bit at runtime (well, the alternatives patching helps here
> but it does not provide compile-time optimization).
> 
> Adding a feature to disabled-features.h allows the bit to be
> checked via a new macro: cpu_feature_enabled().  Note that
> for features in DISABLED_MASK, checks with this macro have
> all of the benefits of an #ifdef.  Before, we would have done
> this in a header:
> 
> #ifdef CONFIG_X86_INTEL_MPX
> #define cpu_has_mpx cpu_has(X86_FEATURE_MPX)
> #else
> #define cpu_has_mpx 0
> #endif
> 
> and this in the code:
> 
> 	if (cpu_has_mpx)
> 		do_some_mpx_thing();
> 
> Now, just add your feature to DISABLED_MASK and you can do this
> everywhere, and get the same benefits you would have from
> #ifdefs:
> 
> 	if (cpu_feature_enabled(X86_FEATURE_MPX))
> 		do_some_mpx_thing();
> 
> We need a new function and *not* a modification to cpu_has()
> because there are cases where we actually need to check the CPU
> itself, despite what features the kernel supports.  The best
> example of this is a hypervisor which has no control over what
> features its guests are using and where the guest does not depend
> on the host for support.
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Acked-by: Borislav Petkov <bp@xxxxxxx>

...

> +/*
> + * Make sure to add features to the correct mask

Hehe, good. :-)

> + */
> +#define DISABLED_MASK0	0
> +#define DISABLED_MASK1	0
> +#define DISABLED_MASK2	0
> +#define DISABLED_MASK3	0
> +#define DISABLED_MASK4	0
> +#define DISABLED_MASK5	0
> +#define DISABLED_MASK6	0
> +#define DISABLED_MASK7	0
> +#define DISABLED_MASK8	0
> +#define DISABLED_MASK9	0
> +
> +#endif /* _ASM_X86_DISABLED_FEATURES_H */
> _
> 

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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