On Thu, Apr 21, 2016 at 5:16 AM, Borislav Petkov <[email protected]> wrote: > On Wed, Apr 20, 2016 at 06:16:01PM -0700, Andy Lutomirski wrote: >> Also, it's time for someone to do UMIP. I'll see if I can convince >> someone in KVM land to emulate it to make it easier to test. > > That'll be fun - we can simply set that bit in CR4 and see who screams > :-P > >> Changes from v1: >> - Remove rdpid() from special_instructions.h. (Was a leftover -- sorry.) >> >> arch/x86/include/asm/cpufeatures.h | 1 + >> arch/x86/include/asm/vgtod.h | 7 ++++++- >> 2 files changed, 7 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/cpufeatures.h >> b/arch/x86/include/asm/cpufeatures.h >> index 7bfb6b70c745..beaf2fb601ee 100644 >> --- a/arch/x86/include/asm/cpufeatures.h >> +++ b/arch/x86/include/asm/cpufeatures.h >> @@ -279,6 +279,7 @@ >> /* Intel-defined CPU features, CPUID level 0x00000007:0 (ecx), word 16 */ >> #define X86_FEATURE_PKU (16*32+ 3) /* Protection Keys for >> Userspace */ >> #define X86_FEATURE_OSPKE (16*32+ 4) /* OS Protection Keys Enable */ >> +#define X86_FEATURE_RDPID (16*32+ 22) /* RDPID instruction */ >> >> /* >> * BUG word(s) >> diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h >> index e728699db774..3a01996db58f 100644 >> --- a/arch/x86/include/asm/vgtod.h >> +++ b/arch/x86/include/asm/vgtod.h >> @@ -89,8 +89,13 @@ static inline unsigned int __getcpu(void) >> * works on all CPUs. This is volatile so that it orders >> * correctly wrt barrier() and to keep gcc from cleverly >> * hoisting it out of the calling function. >> + * >> + * If RDPID is available, use it. >> */ >> - asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); >> + alternative_io ("lsl %[p],%[seg]", >> + ".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */ > > AFAICT, 0xf8 is correct, if I'm reading the SDM right: > > bits [7:6] must be 11b for opcode group 9 and RDPID is in the 11b row, > bits [5:3] are ModRM.reg and they need to be 111b for RDPID (0x7 column) > and the last three [2:0] select the register and they must be 000b for > rAX. > > HOWEVER, you need to make the asm output register constraint "=a" > because you're specifying rAX as a destination register for RDPID.
Didn't I? > > Also, I'm wondering: should we supply that alternative in a separate > inline function in special_instructions.h for wider use? I.e., something > like read_cpu_num() or so... > I thought about it, and there were two reasons: 1. I don't think we want to use __getcpu in the kernel. LSL is fairly slow, and we'd still need to mask off the node number. raw_smp_processor_id(), in contrast, is a single load. 2. I have no way to benchmark this thing. I'm assuming the RDPID will be faster than LSL, but that doesn't mean it's faster than a load. (It could be -- it will save a cache line.) So we might actually want something that does an alternative where the two choices are the percpu load and RDPID ; AND, but that wouldn't end up sharing code. But I'll leave that to someone with an actual RDPID-supporting CPU :) --Andy
