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Subject[RFC PATCH V2 09/22] x86/intel_rdt: Discover supported platforms via prefetch disable bits
Date
Knowing the model specific prefetch disable bits is required to support
cache pseudo-locking because the hardware prefetchers need to be disabled
when the kernel memory is pseudo-locked to cache. We add these bits only
for platforms known to support cache pseudo-locking.

If we have not validated pseudo-locking on a platform that does support
RDT/CAT this should not be seen as a failure of CAT, the pseudo-locking
interface will just not be set up.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c | 80 +++++++++++++++++++++++++++++
1 file changed, 80 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c b/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
index 94bd1b4fbfee..a0c144b5b09b 100644
--- a/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
+++ b/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
@@ -24,8 +24,22 @@
#include <linux/seq_file.h>
#include <linux/stat.h>
#include <linux/slab.h>
+#include <asm/intel-family.h>
#include "intel_rdt.h"

+/*
+ * MSR_MISC_FEATURE_CONTROL register enables the modification of hardware
+ * prefetcher state. Details about this register can be found in the MSR
+ * tables for specific platforms found in Intel's SDM.
+ */
+#define MSR_MISC_FEATURE_CONTROL 0x000001a4
+
+/*
+ * The bits needed to disable hardware prefetching varies based on the
+ * platform. During initialization we will discover which bits to use.
+ */
+static u64 prefetch_disable_bits;
+
struct kernfs_node *pseudo_lock_kn;

/*
@@ -358,6 +372,57 @@ int rdt_pseudo_lock_rmdir(struct kernfs_node *kn)
}

/**
+ * get_prefetch_disable_bits - prefetch disable bits of supported platforms
+ *
+ * Here we capture the list of platforms that have been validated to support
+ * pseudo-locking. This includes testing to ensure pseudo-locked regions
+ * with low cache miss rates can be created under variety of load conditions
+ * as well as that these pseudo-locked regions can maintain their low cache
+ * miss rates under variety of load conditions for significant lengths of time.
+ *
+ * After a platform has been validated to support pseudo-locking its
+ * hardware prefetch disable bits are included here as they are documented
+ * in the SDM.
+ *
+ * RETURNS
+ * If platform is supported, the bits to disable hardware prefetchers, 0
+ * if platform is not supported.
+ */
+static u64 get_prefetch_disable_bits(void)
+{
+ if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+ boot_cpu_data.x86 != 6)
+ return 0;
+
+ switch (boot_cpu_data.x86_model) {
+ case INTEL_FAM6_BROADWELL_X:
+ /*
+ * SDM defines bits of MSR_MISC_FEATURE_CONTROL register
+ * as:
+ * 0 L2 Hardware Prefetcher Disable (R/W)
+ * 1 L2 Adjacent Cache Line Prefetcher Disable (R/W)
+ * 2 DCU Hardware Prefetcher Disable (R/W)
+ * 3 DCU IP Prefetcher Disable (R/W)
+ * 63:4 Reserved
+ */
+ return 0xF;
+ case INTEL_FAM6_ATOM_GOLDMONT:
+ case INTEL_FAM6_ATOM_GEMINI_LAKE:
+ /*
+ * SDM defines bits of MSR_MISC_FEATURE_CONTROL register
+ * as:
+ * 0 L2 Hardware Prefetcher Disable (R/W)
+ * 1 Reserved
+ * 2 DCU Hardware Prefetcher Disable (R/W)
+ * 63:3 Reserved
+ */
+ return 0x5;
+ }
+
+ return 0;
+}
+
+/**
* rdt_pseudo_lock_fs_init - Create and initialize pseudo-locking files
* @root: location in kernfs where directory and files should be created
*
@@ -377,6 +442,17 @@ int rdt_pseudo_lock_fs_init(struct kernfs_node *root)

lockdep_assert_held(&rdtgroup_mutex);

+ /*
+ * Not knowing the bits to disable prefetching is not a failure
+ * that should be propagated since we only return prefetching bits
+ * for those platforms pseudo-locking has been tested on. If
+ * pseudo-locking has not been tested to work on this platform the
+ * other RDT features should continue to be available.
+ */
+ prefetch_disable_bits = get_prefetch_disable_bits();
+ if (prefetch_disable_bits == 0)
+ return 0;
+
pseudo_lock_kn = kernfs_create_dir(root, "pseudo_lock",
root->mode, NULL);
if (IS_ERR(pseudo_lock_kn))
@@ -420,6 +496,10 @@ int rdt_pseudo_lock_fs_init(struct kernfs_node *root)
void rdt_pseudo_lock_fs_remove(void)
{
lockdep_assert_held(&rdtgroup_mutex);
+
+ if (!pseudo_lock_kn)
+ return;
+
mutex_lock(&rdt_pseudo_lock_mutex);

if (new_plr) {
--
2.13.6
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