[PATCH v6 09/12] x86/process: Clear PASID state for a newly forked/cloned thread

Fenghua Yu
2020-07-13T23:49:18Z

The PASID state has to be cleared on forks, since the child has a
different address space. The PASID is also cleared for thread clone. While
it would be correct to inherit the PASID in this case, it is unknown
whether the new task will use ENQCMD. Giving it the PASID "just in case"
would have the downside of increased context switch overhead to setting
the PASID MSR.

Since #GP faults have to be handled on any threads that were created before
the PASID was assigned to the mm of the process, newly created threads
might as well be treated in a consistent way.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
v2:
- Modify init_task_pasid().

arch/x86/kernel/process.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index f362ce0d5ac0..1b1492e337a6 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -121,6 +121,21 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
return do_set_thread_area_64(p, ARCH_SET_FS, tls);
}

+/* Initialize the PASID state for the forked/cloned thread. */
+static void init_task_pasid(struct task_struct *task)
+{
+ struct ia32_pasid_state *ppasid;
+
+ /*
+ * Initialize the PASID state so that the PASID MSR will be
+ * initialized to its initial state (0) by XRSTORS when the task is
+ * scheduled for the first time.
+ */
+ ppasid = get_xsave_addr(&task->thread.fpu.state.xsave, XFEATURE_PASID);
+ if (ppasid)
+ ppasid->pasid = INIT_PASID;
+}
+
int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
unsigned long arg, struct task_struct *p, unsigned long tls)
{
@@ -174,6 +189,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
task_user_gs(p) = get_user_gs(current_pt_regs());
#endif

+ if (static_cpu_has(X86_FEATURE_ENQCMD))
+ init_task_pasid(p);
+
/* Set a new TLS for the child thread? */
if (clone_flags & CLONE_SETTLS)
ret = set_new_tls(p, tls);
--
2.19.1