[PATCH 7/7] x86/process: Clear PASID state for a newly forked/cloned thread

[Fenghua Yu]

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@xxxxxxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/process.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 87de18c64cf5..cefdc8f7fc13 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -122,6 +122,16 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
 		return do_set_thread_area_64(p, ARCH_SET_FS, tls);
 }
 
+/* Clear PASID MSR/state for the forked/cloned thread. */
+static void clear_task_pasid(struct task_struct *task)
+{
+	/*
+	 * Clear the xfeatures bit in the PASID state so that the MSR will be
+	 * initialized to its init state (0) by XRSTORS.
+	 */
+	task->thread.fpu.state.xsave.header.xfeatures &= ~XFEATURE_MASK_PASID;
+}
+
 int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 		    unsigned long arg, struct task_struct *p, unsigned long tls)
 {
@@ -175,6 +185,9 @@ 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))
+		clear_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