lkml.org 
[lkml]   [2019]   [Apr]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH v6 13/20] x86/split_lock: Enable split lock detection by default
On Wed, 3 Apr 2019, Fenghua Yu wrote:

> A split locked access locks bus and degrades overall memory access
> performance. When split lock detection feature is enumerated, enable
> the feature by default to find any split lock issue and then fix
> the issue.

Enabling the feature allows to find the issues, but does not automagically
fix them. Come on.

> +#define DISABLE_SPLIT_LOCK_DETECT 0
> +#define ENABLE_SPLIT_LOCK_DETECT 1

If those defines have a value at all, please start with the facility not
with functionality, i.e. AC_SPLIT_LOCK_ENABLE....

> +
> +static DEFINE_MUTEX(split_lock_detect_mutex);
> +static int split_lock_detect_val;

detect_val? What value is that? Its supposed to hold those magic defines
above. So something like

static unsigned int ac_split_lock_enable;

> /*
> * Just in case our CPU detection goes bad, or you have a weird system,
> * allow a way to override the automatic disabling of MPX.
> @@ -161,10 +167,45 @@ static bool bad_spectre_microcode(struct cpuinfo_x86 *c)
> return false;
> }
>
> +static u32 new_sp_test_ctl_val(u32 test_ctl_val)
> +{
> + /* Change the split lock setting. */
> + if (READ_ONCE(split_lock_detect_val) == DISABLE_SPLIT_LOCK_DETECT)

That READ_ONCE() is required because?

> + test_ctl_val &= ~TEST_CTL_ENABLE_SPLIT_LOCK_DETECT;
> + else
> + test_ctl_val |= TEST_CTL_ENABLE_SPLIT_LOCK_DETECT;
> +
> + return test_ctl_val;
> +}

Aside of that do we really need a misnomed function which replaces the
simple inline code at the call site:

rdmsr(l, h)
l &= ~TEST_CTL_ENABLE_SPLIT_LOCK_DETECT;
l |= ac_split_lock_enable << TEST_CTL_ENABLE_SPLIT_LOCK_DETECT_SHIFT;
wrmrs(...)

or the even more simple

if (ac_split_lock_enable)
msr_set_bit(...)
else
msr_clear_nit(...)

Hmm?

> +
> +static inline void show_split_lock_detection_info(void)
> +{
> + if (READ_ONCE(split_lock_detect_val))

That READ_ONCE() is required because?

> + pr_info_once("x86/split_lock: split lock detection enabled\n");
> + else
> + pr_info_once("x86/split_lock: split lock detection disabled\n");
pr_fmt exists for a reason and having 'split lock' repeated several times
in the same line is not making it more readable.

> +}
> +
> +static void init_split_lock_detect(struct cpuinfo_x86 *c)
> +{
> + if (cpu_has(c, X86_FEATURE_SPLIT_LOCK_DETECT)) {
> + u32 l, h;
> +
> + mutex_lock(&split_lock_detect_mutex);
> + rdmsr(MSR_TEST_CTL, l, h);
> + l = new_sp_test_ctl_val(l);
> + wrmsr(MSR_TEST_CTL, l, h);
> + show_split_lock_detection_info();
> + mutex_unlock(&split_lock_detect_mutex);
> + }
> +}
> +
> static void early_init_intel(struct cpuinfo_x86 *c)
> {
> u64 misc_enable;
>
> + init_split_lock_detect(c);

so we have in early boot:

early_cpu_init()
early_identify_cpu()
this_cpu->c_early_init(c)
early_init_intel() {
init_split_lock_detect();
}
....
cpu_set_core_cap_bits(c)
set(FEATURE_SPLIT_LOCK)
I don't have to understand how init_split_lock_detect() will magically see
the feature bit which gets set afterwards, right?


> +
> /* Unmask CPUID levels if masked: */
> if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
> if (msr_clear_bit(MSR_IA32_MISC_ENABLE,
> @@ -1032,6 +1073,7 @@ cpu_dev_register(intel_cpu_dev);
> static void __init set_split_lock_detect(void)
> {
> setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
> + split_lock_detect_val = 1;

Oh well. You add defines on top of the file and then you don't use them.

Thanks,

tglx

\
 
 \ /
  Last update: 2019-04-04 20:10    [W:0.252 / U:1.400 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site