xen: SMP guest support

This is a fairly straightforward Xen implementation of smp_ops.

Xen has its own IPI mechanisms, and has no dependency on any
APIC-based IPI.  The smp_ops hooks and the flush_tlb_others pv_op
allow a Xen guest to avoid all APIC code in arch/i386 (the only apic
operation is a single apic_read for the apic version number).

One subtle point which needs to be addressed is unpinning pagetables
when another cpu may have a lazy tlb reference to the pagetable. Xen
will not allow an in-use pagetable to be unpinned, so we must find any
other cpus with a reference to the pagetable and get them to shoot
down their references.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andi Kleen <ak@suse.de>

1 files changed, 52 insertions, 17 deletions

diff --git a/arch/i386/xen/mmu.c b/arch/i386/xen/mmu.c
index 53501ce2d15c..bc49ef846203 100644
--- a/arch/i386/xen/mmu.c
+++ b/arch/i386/xen/mmu.c
@@ -391,8 +391,12 @@ void xen_pgd_pin(pgd_t *pgd)
 
         xen_mc_batch();
 
-        if (pgd_walk(pgd, pin_page, TASK_SIZE))
+        if (pgd_walk(pgd, pin_page, TASK_SIZE)) {
+                /* re-enable interrupts for kmap_flush_unused */
+                xen_mc_issue(0);
                 kmap_flush_unused();
+                xen_mc_batch();
+        }
 
         mcs = __xen_mc_entry(sizeof(*op));
         op = mcs.args;
@@ -474,27 +478,58 @@ void xen_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
         spin_unlock(&mm->page_table_lock);
 }
 
-void xen_exit_mmap(struct mm_struct *mm)
-{
-        struct task_struct *tsk = current;
-
-        task_lock(tsk);
 
-        /*
-         * We aggressively remove defunct pgd from cr3. We execute unmap_vmas()
-         * *much* faster this way, as no tlb flushes means bigger wrpt batches.
-         */
-        if (tsk->active_mm == mm) {
-                tsk->active_mm = &init_mm;
-                atomic_inc(&init_mm.mm_count);
+#ifdef CONFIG_SMP
+/* Another cpu may still have their %cr3 pointing at the pagetable, so
+   we need to repoint it somewhere else before we can unpin it. */
+static void drop_other_mm_ref(void *info)
+{
+        struct mm_struct *mm = info;
 
-                switch_mm(mm, &init_mm, tsk);
+        if (__get_cpu_var(cpu_tlbstate).active_mm == mm)
+                leave_mm(smp_processor_id());
+}
 
-                atomic_dec(&mm->mm_count);
-                BUG_ON(atomic_read(&mm->mm_count) == 0);
+static void drop_mm_ref(struct mm_struct *mm)
+{
+        if (current->active_mm == mm) {
+                if (current->mm == mm)
+                        load_cr3(swapper_pg_dir);
+                else
+                        leave_mm(smp_processor_id());
         }
 
-        task_unlock(tsk);
+        if (!cpus_empty(mm->cpu_vm_mask))
+                xen_smp_call_function_mask(mm->cpu_vm_mask, drop_other_mm_ref,
+                                           mm, 1);
+}
+#else
+static void drop_mm_ref(struct mm_struct *mm)
+{
+        if (current->active_mm == mm)
+                load_cr3(swapper_pg_dir);
+}
+#endif
+
+/*
+ * While a process runs, Xen pins its pagetables, which means that the
+ * hypervisor forces it to be read-only, and it controls all updates
+ * to it.  This means that all pagetable updates have to go via the
+ * hypervisor, which is moderately expensive.
+ *
+ * Since we're pulling the pagetable down, we switch to use init_mm,
+ * unpin old process pagetable and mark it all read-write, which
+ * allows further operations on it to be simple memory accesses.
+ *
+ * The only subtle point is that another CPU may be still using the
+ * pagetable because of lazy tlb flushing.  This means we need need to
+ * switch all CPUs off this pagetable before we can unpin it.
+ */
+void xen_exit_mmap(struct mm_struct *mm)
+{
+        get_cpu();              /* make sure we don't move around */
+        drop_mm_ref(mm);
+        put_cpu();
 
         xen_pgd_unpin(mm->pgd);
 }