[PATCH PATCH 7/9] x86/cpu: Update x86_match_cpu() to also use cpu-type

Pawan Gupta
2024-06-17T09:12:03Z

Non-hybrid CPU variants that share the same Family/Model could be
differentiated by their cpu-type. x86_match_cpu() currently does not use
cpu-type for CPU matching.

Dave Hansen suggested to use below conditions to match CPU-type:

1. If CPU_TYPE_ANY (the wildcard), then matched
2. If hybrid, then matched
3. If !hybrid, look at the boot CPU and compare the cpu-type to determine
if it is a match.

This special case for hybrid systems allows more compact vulnerability
list. Imagine that "Haswell" CPUs might or might not be hybrid and that
only Atom cores are vulnerable to Meltdown. That means there are three
possibilities:

1. P-core only
2. Atom only
3. Atom + P-core (aka. hybrid)

One might be tempted to code up the vulnerability list like this:

MATCH( HASWELL, X86_FEATURE_HYBRID, MELTDOWN)
MATCH_TYPE(HASWELL, ATOM, MELTDOWN)

Logically, this matches #2 and #3. But that's a little silly. You would
only ask for the "ATOM" match in cases where there *WERE* hybrid cores in
play. You shouldn't have to _also_ ask for hybrid cores explicitly.

In short, assume that processors that enumerate Hybrid==1 have a
vulnerable core type.

Update x86_match_cpu() to also match cpu-type. Also treat hybrid systems as
special, and match them to any cpu-type.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
arch/x86/kernel/cpu/match.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)

diff --git a/arch/x86/kernel/cpu/match.c b/arch/x86/kernel/cpu/match.c
index 8e7de733320a..ca15e74596d7 100644
--- a/arch/x86/kernel/cpu/match.c
+++ b/arch/x86/kernel/cpu/match.c
@@ -5,6 +5,26 @@
#include <linux/export.h>
#include <linux/slab.h>

+/**
+ * x86_match_cpu - helper function to match the cpu-type for a single
+ * entry in the x86_cpu_id table.
+ * @c: Pointer to the cpuinfo_x86 structure of the CPU to match.
+ * @m: Pointer to the x86_cpu_id entry to match against.
+ *
+ * Return: true if the cpu-type matches, false otherwise.
+ */
+static bool x86_match_cpu_type(struct cpuinfo_x86 *c, const struct x86_cpu_id *m)
+{
+ if (m->cpu_type == X86_CPU_TYPE_ANY)
+ return true;
+
+ /* Hybrid CPUs are special, they are assumed to match all cpu-types */
+ if (boot_cpu_has(X86_FEATURE_HYBRID_CPU))
+ return true;
+
+ return c->topo.cpu_type == m->cpu_type;
+}
+
/**
* x86_match_cpu - match current CPU again an array of x86_cpu_ids
* @match: Pointer to array of x86_cpu_ids. Last entry terminated with
@@ -50,6 +70,8 @@ const struct x86_cpu_id *x86_match_cpu(const struct x86_cpu_id *match)
continue;
if (m->feature != X86_FEATURE_ANY && !cpu_has(c, m->feature))
continue;
+ if (!x86_match_cpu_type(c, m))
+ continue;
return m;
}
return NULL;

--
2.34.1