s390: KVM preparation: provide hook to enable pgstes in user pagetable

The SIE instruction on s390 uses the 2nd half of the page table page to
virtualize the storage keys of a guest. This patch offers the s390_enable_sie
function, which reorganizes the page tables of a single-threaded process to
reserve space in the page table:
s390_enable_sie makes sure that the process is single threaded and then uses
dup_mm to create a new mm with reorganized page tables. The old mm is freed
and the process has now a page status extended field after every page table.

Code that wants to exploit pgstes should SELECT CONFIG_PGSTE.

This patch has a small common code hit, namely making dup_mm non-static.

Edit (Carsten): I've modified Martin's patch, following Jeremy Fitzhardinge's
review feedback. Now we do have the prototype for dup_mm in
include/linux/sched.h. Following Martin's suggestion, s390_enable_sie() does now
call task_lock() to prevent race against ptrace modification of mm_users.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Avi Kivity <avi@qumranet.com>

3 files changed, 70 insertions, 3 deletions

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index f6a68e178fc5..513a0589e81d 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -62,6 +62,10 @@ config GENERIC_LOCKBREAK
         default y
         depends on SMP && PREEMPT
 
+config PGSTE
+        bool
+        default y if KVM
+
 mainmenu "Linux Kernel Configuration"
 
 config S390
diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index 7141147e6b63..2f35133ebc18 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -316,7 +316,11 @@ static int __init early_parse_ipldelay(char *p)
 early_param("ipldelay", early_parse_ipldelay);
 
 #ifdef CONFIG_S390_SWITCH_AMODE
+#ifdef CONFIG_PGSTE
+unsigned int switch_amode = 1;
+#else
 unsigned int switch_amode = 0;
+#endif
 EXPORT_SYMBOL_GPL(switch_amode);
 
 static void set_amode_and_uaccess(unsigned long user_amode,
diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index fd072013f88c..5c1aea97cd12 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -30,11 +30,27 @@
 #define TABLES_PER_PAGE 4
 #define FRAG_MASK       15UL
 #define SECOND_HALVES   10UL
+
+void clear_table_pgstes(unsigned long *table)
+{
+        clear_table(table, _PAGE_TYPE_EMPTY, PAGE_SIZE/4);
+        memset(table + 256, 0, PAGE_SIZE/4);
+        clear_table(table + 512, _PAGE_TYPE_EMPTY, PAGE_SIZE/4);
+        memset(table + 768, 0, PAGE_SIZE/4);
+}
+
 #else
 #define ALLOC_ORDER     2
 #define TABLES_PER_PAGE 2
 #define FRAG_MASK       3UL
 #define SECOND_HALVES   2UL
+
+void clear_table_pgstes(unsigned long *table)
+{
+        clear_table(table, _PAGE_TYPE_EMPTY, PAGE_SIZE/2);
+        memset(table + 256, 0, PAGE_SIZE/2);
+}
+
 #endif
 
 unsigned long *crst_table_alloc(struct mm_struct *mm, int noexec)
@@ -153,7 +169,7 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
         unsigned long *table;
         unsigned long bits;
 
-        bits = mm->context.noexec ? 3UL : 1UL;
+        bits = (mm->context.noexec || mm->context.pgstes) ? 3UL : 1UL;
         spin_lock(&mm->page_table_lock);
         page = NULL;
         if (!list_empty(&mm->context.pgtable_list)) {
@@ -170,7 +186,10 @@ unsigned long *page_table_alloc(struct mm_struct *mm)
                 pgtable_page_ctor(page);
                 page->flags &= ~FRAG_MASK;
                 table = (unsigned long *) page_to_phys(page);
-                clear_table(table, _PAGE_TYPE_EMPTY, PAGE_SIZE);
+                if (mm->context.pgstes)
+                        clear_table_pgstes(table);
+                else
+                        clear_table(table, _PAGE_TYPE_EMPTY, PAGE_SIZE);
                 spin_lock(&mm->page_table_lock);
                 list_add(&page->lru, &mm->context.pgtable_list);
         }
@@ -191,7 +210,7 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
         struct page *page;
         unsigned long bits;
 
-        bits = mm->context.noexec ? 3UL : 1UL;
+        bits = (mm->context.noexec || mm->context.pgstes) ? 3UL : 1UL;
         bits <<= (__pa(table) & (PAGE_SIZE - 1)) / 256 / sizeof(unsigned long);
         page = pfn_to_page(__pa(table) >> PAGE_SHIFT);
         spin_lock(&mm->page_table_lock);
@@ -228,3 +247,43 @@ void disable_noexec(struct mm_struct *mm, struct task_struct *tsk)
         mm->context.noexec = 0;
         update_mm(mm, tsk);
 }
+
+/*
+ * switch on pgstes for its userspace process (for kvm)
+ */
+int s390_enable_sie(void)
+{
+        struct task_struct *tsk = current;
+        struct mm_struct *mm;
+        int rc;
+
+        task_lock(tsk);
+
+        rc = 0;
+        if (tsk->mm->context.pgstes)
+                goto unlock;
+
+        rc = -EINVAL;
+        if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 ||
+            tsk->mm != tsk->active_mm || tsk->mm->ioctx_list)
+                goto unlock;
+
+        tsk->mm->context.pgstes = 1;    /* dirty little tricks .. */
+        mm = dup_mm(tsk);
+        tsk->mm->context.pgstes = 0;
+
+        rc = -ENOMEM;
+        if (!mm)
+                goto unlock;
+        mmput(tsk->mm);
+        tsk->mm = tsk->active_mm = mm;
+        preempt_disable();
+        update_mm(mm, tsk);
+        cpu_set(smp_processor_id(), mm->cpu_vm_mask);
+        preempt_enable();
+        rc = 0;
+unlock:
+        task_unlock(tsk);
+        return rc;
+}
+EXPORT_SYMBOL_GPL(s390_enable_sie);