[Xen-devel] Unreachable code about cpu features

[Choonho Son <choonho.son@xxxxxxxxx>]

Hi all,

I still have some questions about cpu flags.

- Reference: http://lists.xen.org/archives/html/xen-devel/2013-03/msg00891.html

I printed the values about following code:

 if (!~(opt_cpuid_mask_ecx & opt_cpuid_mask_edx &

               opt_cpuid_mask_ext_ecx & opt_cpuid_mask_ext_edx &

               opt_cpuid_mask_xsave_eax))

                return;

Above code is always true, so the next "switch" code is not reachable.

Is it correct code?

##############################

# Source: xen/arch/x86/cpu/intel.c

##############################

#################

# debugging result

#################

(XEN) opt_cpuid_mask_ecx:-1 opt_cpuid_mask_edx:-1 opt_cpuid_mask_ext_ecx:-1 opt_cpuid_mask_ext_edx:-1 opt_cpuid_mask_xsave_eax:-1

/*

 * opt_cpuid_mask_ecx/edx: cpuid.1[ecx, edx] feature mask.

 * For example, E8400[Intel Core 2 Duo Processor series] ecx = 0x0008E3FD,

 * edx = 0xBFEBFBFF when executing CPUID.EAX = 1 normally. If you want to

 * 'rev down' to E8400, you can set these values in these Xen boot parameters.

 */

static void __devinit set_cpuidmask(const struct cpuinfo_x86 *c)

{

        u32 eax, edx;

        const char *extra = "";

        printk(XENLOG_INFO  "opt_cpuid_mask_ecx:%d opt_cpuid_mask_edx:%d opt_cpuid_mask_ext_ecx:%d opt_cpuid_mask_ext_edx:%d opt_cpuid_mask_xsave_eax:%d\n", opt_cpuid_mask_ecx, opt_cpuid_mask_edx, opt_cpuid_mask_ext_ecx, opt_cpuid_mask_ext_edx,opt_cpuid_mask_xsave_eax);

        if (!~(opt_cpuid_mask_ecx & opt_cpuid_mask_edx &

               opt_cpuid_mask_ext_ecx & opt_cpuid_mask_ext_edx &

               opt_cpuid_mask_xsave_eax))

                return;

        /**************************

         * Unreachable code (?)

         **************************/

        /* Only family 6 supports this feature  */

        switch ((c->x86 == 6) * c->x86_model) {

        case 0x17:

                if ((c->x86_mask & 0x0f) < 4)

                        break;

                /* fall through */

        case 0x1d:

                wrmsr(MSR_INTEL_CPUID_FEATURE_MASK,

         ...

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