c - WRMSR on x86_64 64bit RCX register value is wrongly set -


i want write pmc1 register, want set rcx 188.

the code use use wrmsr instruction attached @ end. problem pass eax , ecx value (64bit) macro, but %rcx register set eax value.

#define rtxen_write_msr(eax, ecx)     __asm__ __volatile__(\                                 "movq %0, %%rax\n\t"\                                 "xorq %%rdx, %%rdx\n\t"\                                 "xorq %%rcx, %%rcx\n\t"\                                 "movq %1, %%rcx\n\t"\                                 "wrmsr"\                                 :\                                 :"r" (eax), "r" (ecx)\                                 :\                                 )   uint64_t eax = 0x14f2e uint64_t edx = 0x188  printk("eax:%#018lx, edx:%#018lx\n", eax, edx); rtxen_write_msr(eax, ecx);

when rtxen_write_msr(eax, ecx) executed, kernel panic! register information follows:

(xen) cpu:    1 (xen) rip:    e008:[<ffff82c4c02166b1>] setread_perf_counter+0x251/0x4c0 (xen) rflags: 0000000000010046   context: hypervisor (xen) rax: 0000000000014f2e   rbx: ffff8308558f7718   **rcx: 0000000000014f2e** (xen) rdx: 0000000000000000   rsi: 000000000000000a   rdi: ffff82c4c0270640 (xen) rbp: 0000000000000001   rsp: ffff83086bec7cd8   r8:  0000000000000004 (xen) r9:  0000000000000004   r10: 0000000000000004   r11: 0000000000000400 (xen) r12: 0000000000014f2e   r13: 0000000200000000   r14: ffff83086bec7dd8 (xen) r15: 00000000000000fb   cr0: 000000008005003b   cr4: 00000000001426f0 (xen) cr3: 000000008ce6a000   cr2: ffff880115ca32a0 (xen) ds: 002b   es: 002b   fs: 0000   gs: 0000   ss: e010   cs: e008

can me see what's wrong in code?

you know can have compiler pass arguments in right registers straight away?

asm volatile (     "wrmsr"     :     : "a" ((uint32_t)eax)     , "c" ((uint32_t)ecx)     , "d" ((uint32_t)0) );

the typecast uint32_t ensures compiler use 32-bit register variants (eax, ecx, edx).

you can find constrain modifiers intel registers here: http://gcc.gnu.org/onlinedocs/gcc/machine-constraints.html#machine-constraints

on side note: if using registers in asm need tell compiler doing so. need put registers use , compiler doen't know clobber registers list after third colon.


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