Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 27 Oct 2016 15:34:53 -0700 | Subject | Re: [PATCH v7 6/6] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID |
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On Thu, Oct 27, 2016 at 4:15 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > This is insane. The compiler makes that a conditional jump and then in > switch_cpuid_faulting we get another one. Further switch_cpuid_faulting() > calls into lib/msr which is adding even more overhead. > > msr_set/clear_bit() are nice for random driver code, but complete overkill > for the context switch hotpath. > > That's just not acceptable for switch_to(). We keep adding cruft and then > wonder why context switches slow down despite machines getting faster. > > This can and needs to be done smarter. See untested patch below. The > resulting code has a single conditional jump, which is obviously the check > for a change between prev and next. Everything else is done with straight > linear shift,add,and,rdmsr,wrmsr instructions. >
...
> #define MSR_IA32_SYSENTER_ESP 0x00000175 > --- a/arch/x86/kernel/process.c > +++ b/arch/x86/kernel/process.c > @@ -193,12 +193,17 @@ int set_tsc_mode(unsigned int val) > return 0; > } > > -static void switch_cpuid_faulting(bool on) > +#define CPUID_FAULT_ON_MASK (~0ULL) > +#define CPUID_FAULT_OFF_MASK (~CPUID_FAULT_ENABLE) > + > +static void cpuid_fault_ctrl(u64 msk) > { > - if (on) > - msr_set_bit(MSR_MISC_FEATURES_ENABLES, 0); > - else > - msr_clear_bit(MSR_MISC_FEATURES_ENABLES, 0); > + u64 msrval; > + > + rdmsrl(MSR_MISC_FEATURES_ENABLES, msrval); > + msrval |= CPUID_FAULT_ENABLE; > + msrval &= msk; > + wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval); > }
Let's just do this right from day one:
static void set_cpuid_faulting(bool on) { u64 msrval;
DEBUG_LOCKS_WARN_ON(!irqs_disabled());
msrval = this_cpu_read(msr_misc_features_enables_shadow); msrval &= CPUID_FAULT_ENABLE; msrval |= (on << CPUID_FAULT_ENABLE_BIT); this_cpu_write(msr_misc_features_enables_shadow, msrval); wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval); }
RDMSR may be considerably faster than WRMSR, but that doesn't mean it's *fast*.
Obviously this needs some initialization code, but that's fine IMO.
--Andy
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