* Kyle Huey <me@xxxxxxxxxxxx> wrote: > rr (http://rr-project.org/), a userspace record-and-replay reverse- > execution debugger, would like to trap and emulate the CPUID instruction. > This would allow us to a) mask away certain hardware features that rr does > not support (e.g. RDRAND) and b) enable trace portability across machines > by providing constant results. > > Newer Intel CPUs (Ivy Bridge and later) can fault when CPUID is executed at > CPL > 0. Expose this capability to userspace as a new pair of arch_prctls, > ARCH_GET_CPUID and ARCH_SET_CPUID. > > Since v12: > Patch 4: x86/syscalls/32: Wire up arch_prctl on x86-32 > - compat_sys_arch_prctl prototype has argument names. So while I am fine with the feature, I'm still unconvinced about the implementation: 1) As I pointed out before, the arbitrary 'code' argument name x86-ism should be changed to 'option' like the canonical core kernel option name is for prctls(). This is still unfixed. 2) As I complained about in my first review, TIF_NOCPUID flag is too far removed from the value what will be written into the MSR. The result is poor code generation on 64-bit defconfig+CONFIG_PREEMPT=y: if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^ test_tsk_thread_flag(next_p, TIF_NOCPUID)) { set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID)); is compiled as: 476: 49 8b 06 mov (%r14),%rax 479: 49 8b 55 00 mov 0x0(%r13),%rdx 47d: 48 c1 e8 0f shr $0xf,%rax 481: 48 c1 ea 0f shr $0xf,%rdx 485: 83 e2 01 and $0x1,%edx 488: 83 e0 01 and $0x1,%eax 48b: 38 c2 cmp %al,%dl 48d: 74 10 je 49f <__switch_to_xtra+0x9f> 48f: 49 8b 7d 00 mov 0x0(%r13),%rdi 493: 48 c1 ef 0f shr $0xf,%rdi 497: 83 e7 01 and $0x1,%edi 49a: e8 61 fb ff ff callq 0 <set_cpuid_faulting> ... the first 7 instructions burdens all __switch_to_xtra() users, not just the faulting-CPUID users. The set_cpuid_faulting() call is also unnecessary and the set_cpuid_faulting() call generates into an obscene sequence of: 0000000000000000 <set_cpuid_faulting>: 0: 8b 15 00 00 00 00 mov 0x0(%rip),%edx # 6 <set_cpuid_faulting+0x6> 6: 55 push %rbp 7: 48 89 e5 mov %rsp,%rbp a: 53 push %rbx b: 40 0f b6 df movzbl %dil,%ebx f: 85 d2 test %edx,%edx 11: 75 07 jne 1a <set_cpuid_faulting+0x1a> 13: 9c pushfq 14: 58 pop %rax 15: f6 c4 02 test $0x2,%ah 18: 75 48 jne 62 <set_cpuid_faulting+0x62> 1a: 65 48 8b 05 00 00 00 mov %gs:0x0(%rip),%rax # 22 <set_cpuid_faulting+0x22> 21: 00 22: 48 83 e0 fe and $0xfffffffffffffffe,%rax 26: b9 40 01 00 00 mov $0x140,%ecx 2b: 48 09 d8 or %rbx,%rax 2e: 48 89 c2 mov %rax,%rdx 31: 65 48 89 05 00 00 00 mov %rax,%gs:0x0(%rip) # 39 <set_cpuid_faulting+0x39> 38: 00 39: 48 c1 ea 20 shr $0x20,%rdx 3d: 0f 30 wrmsr 3f: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 44: 5b pop %rbx 45: 5d pop %rbp 46: c3 retq 47: 48 c1 e2 20 shl $0x20,%rdx 4b: 89 c0 mov %eax,%eax 4d: bf 40 01 00 00 mov $0x140,%edi 52: 48 09 d0 or %rdx,%rax 55: 31 d2 xor %edx,%edx 57: 48 89 c6 mov %rax,%rsi 5a: e8 00 00 00 00 callq 5f <set_cpuid_faulting+0x5f> 5f: 5b pop %rbx 60: 5d pop %rbp 61: c3 retq 62: e8 00 00 00 00 callq 67 <set_cpuid_faulting+0x67> 67: 85 c0 test %eax,%eax 69: 74 af je 1a <set_cpuid_faulting+0x1a> 6b: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 71 <set_cpuid_faulting+0x71> 71: 85 c0 test %eax,%eax 73: 75 a5 jne 1a <set_cpuid_faulting+0x1a> 75: 48 c7 c1 00 00 00 00 mov $0x0,%rcx 7c: 48 c7 c2 00 00 00 00 mov $0x0,%rdx 83: be b9 00 00 00 mov $0xb9,%esi 88: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 8f: e8 00 00 00 00 callq 94 <set_cpuid_faulting+0x94> 94: eb 84 jmp 1a <set_cpuid_faulting+0x1a> 96: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1) 9d: 00 00 00 The affected object file code size blows up as well, by 17%: arch/x86/kernel/process.o: text data bss dec hex filename 3325 8577 32 11934 2e9e process.o.before 3889 8609 32 12530 30f2 process.o.after A good deal of this overhead and complexity comes from the implementation inefficiency I pointed out, and all this can be avoided with the method I suggested in my previous review, by caching the per task MSR value in the thread struct. So sorry, NAK for this implementation - especially considering how relatively straightforward the changes I suggested are to implement. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
