On Tue, Jul 08, 2014 at 09:49:40AM -0700, kan.liang@intel.com wrote:
> --- a/arch/x86/kernel/cpu/perf_event.h
> +++ b/arch/x86/kernel/cpu/perf_event.h
> @@ -464,6 +464,12 @@ struct x86_pmu {
>  	 */
>  	struct extra_reg *extra_regs;
>  	unsigned int er_flags;
> +	/*
> +	 * EXTRA REG MSR can be accessed
> +	 * The extra registers are completely unrelated to each other.
> +	 * So it needs a flag for each extra register.
> +	 */
> +	bool		extra_msr_access[EXTRA_REG_MAX];
>  
>  	/*
>  	 * Intel host/guest support (KVM)

# pahole -C extra_reg arch/x86/kernel/cpu/perf_event_intel.o
struct extra_reg {
        unsigned int               event;                /*     0     4 */
        unsigned int               msr;                  /*     4     4 */
        u64                        config_mask;          /*     8     8 */
        u64                        valid_mask;           /*    16     8 */
        int                        idx;                  /*    24     4 */

        /* size: 32, cachelines: 1, members: 5 */
        /* padding: 4 */
        /* last cacheline: 32 bytes */
};

There's still 4 empty bytes at the tail of extra_reg itself; would it
make sense to store the availability of the reg in there?

After all; the place we use it (x86_pmu_extra_regs) already has the
pointer to the structure.