Re: [Xen-devel] [PATCH v2 08/30] xen/x86: Mask out unknown features from Xen's capabilities

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/02/16 16:43, Jan Beulich wrote:
>>>> On 05.02.16 at 14:42, <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- /dev/null
>> +++ b/xen/arch/x86/cpuid.c
>> @@ -0,0 +1,19 @@
>> +#include <xen/lib.h>
>> +#include <asm/cpuid.h>
>> +
>> +const uint32_t known_features[] = INIT_KNOWN_FEATURES;
>> +
>> +static void __maybe_unused build_assertions(void)
>> +{
>> +    BUILD_BUG_ON(ARRAY_SIZE(known_features) != FSCAPINTS);
> This is sort of redundant with ...
>
>> --- /dev/null
>> +++ b/xen/include/asm-x86/cpuid.h
>> @@ -0,0 +1,24 @@
>> +#ifndef __X86_CPUID_H__
>> +#define __X86_CPUID_H__
>> +
>> +#include <asm/cpuid-autogen.h>
>> +
>> +#define FSCAPINTS FEATURESET_NR_ENTRIES
>> +
>> +#ifndef __ASSEMBLY__
>> +#include <xen/types.h>
>> +
>> +extern const uint32_t known_features[FSCAPINTS];
> ... the use of FSCAPINTS here. You'd catch more mistakes if you
> just used [] here.

Not quite.

The extern gives an explicit size so other translation units can use
ARRAY_SIZE().

Without the BUILD_BUG_ON(), const uint32_t known_features[] can actually
be longer than FSCAPINTS, and everything compiles fine.

The BUILD_BUG_ON() were introduced following an off-by-one error
generating INIT_KNOWN_FEATURES, where ARRAY_SIZE(known_features) was
different in this translation unit than all others.

>
> But either way
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks,

~Andrew

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