[Devel] [PATCH RHEL8 COMMIT] x86: don't enable cpuid faults if /proc/vz/cpuid_override unused #PSBM-121823

[Konstantin Khorenko]

The commit is pushed to "work" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-193.6.3.vz8.4.16
------>
commit b82cdbd9074b894fd5f0f6f098c017e5c5a64ba8
Author: Andrey Ryabinin <aryabinin at virtuozzo.com>
Date:   Fri Nov 6 17:11:44 2020 +0300

    x86: don't enable cpuid faults if /proc/vz/cpuid_override unused #PSBM-121823
    
    We don't need to enable cpuid faults if /proc/vz/cpuid_override
    was never used. If task was attached to ve before a write to
    'cpuid_override' it will not get cpuid faults now. It shouldn't
    be a problem since the proper use of 'cpuid_override' requires
    stopping all containers.
    
    https://jira.sw.ru/browse/PSBM-121823
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
    Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 arch/x86/kernel/cpuid_fault.c  | 21 ++-------------------
 include/linux/cpuid_override.h | 30 ++++++++++++++++++++++++++++++
 kernel/ve/ve.c                 |  5 ++++-
 3 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kernel/cpuid_fault.c b/arch/x86/kernel/cpuid_fault.c
index 1e8ffacc4412..cb6c2216fa8a 100644
--- a/arch/x86/kernel/cpuid_fault.c
+++ b/arch/x86/kernel/cpuid_fault.c
@@ -1,3 +1,4 @@
+#include <linux/cpuid_override.h>
 #include <linux/gfp.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
@@ -9,25 +10,7 @@
 #include <linux/veowner.h>
 #include <linux/uaccess.h>
 
-struct cpuid_override_entry {
-	unsigned int op;
-	unsigned int count;
-	bool has_count;
-	unsigned int eax;
-	unsigned int ebx;
-	unsigned int ecx;
-	unsigned int edx;
-};
-
-#define MAX_CPUID_OVERRIDE_ENTRIES	16
-
-struct cpuid_override_table {
-	struct rcu_head rcu_head;
-	int size;
-	struct cpuid_override_entry entries[MAX_CPUID_OVERRIDE_ENTRIES];
-};
-
-static struct cpuid_override_table __rcu *cpuid_override __read_mostly;
+struct cpuid_override_table __rcu *cpuid_override __read_mostly;
 static DEFINE_SPINLOCK(cpuid_override_lock);
 
 static void cpuid_override_update(struct cpuid_override_table *new_table)
diff --git a/include/linux/cpuid_override.h b/include/linux/cpuid_override.h
new file mode 100644
index 000000000000..ea0fa7af3d3c
--- /dev/null
+++ b/include/linux/cpuid_override.h
@@ -0,0 +1,30 @@
+#ifndef __CPUID_OVERRIDE_H
+#define __CPUID_OVERRIDE_H
+
+#include <linux/rcupdate.h>
+
+struct cpuid_override_entry {
+	unsigned int op;
+	unsigned int count;
+	bool has_count;
+	unsigned int eax;
+	unsigned int ebx;
+	unsigned int ecx;
+	unsigned int edx;
+};
+
+#define MAX_CPUID_OVERRIDE_ENTRIES	16
+
+struct cpuid_override_table {
+	struct rcu_head rcu_head;
+	int size;
+	struct cpuid_override_entry entries[MAX_CPUID_OVERRIDE_ENTRIES];
+};
+
+extern struct cpuid_override_table __rcu *cpuid_override;
+
+static inline bool cpuid_override_on(void)
+{
+	return rcu_access_pointer(cpuid_override);
+}
+#endif
diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
index a6e3a145d51d..a9afefc5b9de 100644
--- a/kernel/ve/ve.c
+++ b/kernel/ve/ve.c
@@ -9,6 +9,7 @@
  * 've.c' helper file performing VE sub-system initialization
  */
 
+#include <linux/cpuid_override.h>
 #include <linux/ctype.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
@@ -801,6 +802,7 @@ static void ve_attach(struct cgroup_taskset *tset)
 {
 	struct cgroup_subsys_state *css;
 	struct task_struct *task;
+	extern struct cpuid_override_table __rcu *cpuid_override;
 
 	cgroup_taskset_for_each(task, css, tset) {
 		struct ve_struct *ve = css_to_ve(css);
@@ -816,7 +818,8 @@ static void ve_attach(struct cgroup_taskset *tset)
 		/* Leave parent exec domain */
 		task->parent_exec_id--;
 
-		set_tsk_thread_flag(task, TIF_CPUID_OVERRIDE);
+		if (cpuid_override_on())
+			set_tsk_thread_flag(task, TIF_CPUID_OVERRIDE);
 		task->task_ve = ve;
 	}
 }