lkml.org 
[lkml]   [2016]   [Oct]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH v7 6/6] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID
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

\
 
 \ /
  Last update: 2016-10-28 00:35    [W:0.033 / U:7.596 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site