File 5d03a0c4-D-use-guest-atomics-helpers.patch of Package xen

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File 5d03a0c4-D-use-guest-atomics-helpers.patch of Package xen (Revision 1feb9822b6008e55083152e7093e909f)

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# Commit 4b0c004beb22777572c1b8bfd1404caddfb268f0
# Date 2019-06-14 14:27:32 +0100
# Author Julien Grall <julien.grall@arm.com>
# Committer Julien Grall <julien.grall@arm.com>
xen: Use guest atomics helpers when modifying atomically guest memory

On Arm, exclusive load-store atomics should only be used between trusted
thread. As not all the guests are trusted, it may be possible to DoS Xen
when updating shared memory with guest atomically.

This patch replaces all the atomics operations on shared memory with
a guest by the new guest atomics helpers. The x86 code was not audited
to know where guest atomics helpers could be used. I will leave that
to the x86 folks.

Note that some rework was required in order to plumb use the new guest
atomics in event channel and grant-table.

Because guest_test_bit is ignoring the parameter "d" for now, it
means there a lot of places do not need to drop the const. We may want
to revisit this in the future if the parameter "d" becomes necessary.

This is part of XSA-295.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -27,6 +27,7 @@
 #include <asm/event.h>
 #include <asm/gic.h>
 #include <asm/guest_access.h>
+#include <asm/guest_atomics.h>
 #include <asm/irq.h>
 #include <asm/p2m.h>
 #include <asm/platform.h>
@@ -1017,7 +1018,7 @@ void arch_dump_vcpu_info(struct vcpu *v)
 
 void vcpu_mark_events_pending(struct vcpu *v)
 {
-    int already_pending = test_and_set_bit(
+    bool already_pending = guest_test_and_set_bit(v->domain,
         0, (unsigned long *)&vcpu_info(v, evtchn_upcall_pending));
 
     if ( already_pending )
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -40,6 +40,8 @@
 #include <xen/pfn.h>
 #include <xen/sizes.h>
 #include <xen/libfdt/libfdt.h>
+
+#include <asm/guest_atomics.h>
 #include <asm/setup.h>
 
 struct domain *dom_xen, *dom_io, *dom_cow;
@@ -1380,7 +1382,7 @@ void put_page_type(struct page_info *pag
     return;
 }
 
-void gnttab_clear_flag(unsigned long nr, uint16_t *addr)
+void gnttab_clear_flag(struct domain *d, unsigned long nr, uint16_t *addr)
 {
     /*
      * Note that this cannot be clear_bit(), as the access must be
@@ -1390,7 +1392,7 @@ void gnttab_clear_flag(unsigned long nr,
 
     do {
         old = *addr;
-    } while (cmpxchg(addr, old, old & mask) != old);
+    } while (guest_cmpxchg(d, addr, old, old & mask) != old);
 }
 
 void gnttab_mark_dirty(struct domain *d, mfn_t mfn)
--- a/xen/common/event_2l.c
+++ b/xen/common/event_2l.c
@@ -13,6 +13,8 @@
 #include <xen/sched.h>
 #include <xen/event.h>
 
+#include <asm/guest_atomics.h>
+
 static void evtchn_2l_set_pending(struct vcpu *v, struct evtchn *evtchn)
 {
     struct domain *d = v->domain;
@@ -25,12 +27,12 @@ static void evtchn_2l_set_pending(struct
      * others may require explicit memory barriers.
      */
 
-    if ( test_and_set_bit(port, &shared_info(d, evtchn_pending)) )
+    if ( guest_test_and_set_bit(d, port, &shared_info(d, evtchn_pending)) )
         return;
 
-    if ( !test_bit        (port, &shared_info(d, evtchn_mask)) &&
-         !test_and_set_bit(port / BITS_PER_EVTCHN_WORD(d),
-                           &vcpu_info(v, evtchn_pending_sel)) )
+    if ( !guest_test_bit(d, port, &shared_info(d, evtchn_mask)) &&
+         !guest_test_and_set_bit(d, port / BITS_PER_EVTCHN_WORD(d),
+                                 &vcpu_info(v, evtchn_pending_sel)) )
     {
         vcpu_mark_events_pending(v);
     }
@@ -40,7 +42,7 @@ static void evtchn_2l_set_pending(struct
 
 static void evtchn_2l_clear_pending(struct domain *d, struct evtchn *evtchn)
 {
-    clear_bit(evtchn->port, &shared_info(d, evtchn_pending));
+    guest_clear_bit(d, evtchn->port, &shared_info(d, evtchn_pending));
 }
 
 static void evtchn_2l_unmask(struct domain *d, struct evtchn *evtchn)
@@ -52,10 +54,10 @@ static void evtchn_2l_unmask(struct doma
      * These operations must happen in strict order. Based on
      * evtchn_2l_set_pending() above.
      */
-    if ( test_and_clear_bit(port, &shared_info(d, evtchn_mask)) &&
-         test_bit          (port, &shared_info(d, evtchn_pending)) &&
-         !test_and_set_bit (port / BITS_PER_EVTCHN_WORD(d),
-                            &vcpu_info(v, evtchn_pending_sel)) )
+    if ( guest_test_and_clear_bit(d, port, &shared_info(d, evtchn_mask)) &&
+         guest_test_bit(d, port, &shared_info(d, evtchn_pending)) &&
+         !guest_test_and_set_bit(d, port / BITS_PER_EVTCHN_WORD(d),
+                                 &vcpu_info(v, evtchn_pending_sel)) )
     {
         vcpu_mark_events_pending(v);
     }
@@ -66,7 +68,8 @@ static bool evtchn_2l_is_pending(const s
     unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
 
     ASSERT(port < max_ports);
-    return port < max_ports && test_bit(port, &shared_info(d, evtchn_pending));
+    return (port < max_ports &&
+            guest_test_bit(d, port, &shared_info(d, evtchn_pending)));
 }
 
 static bool evtchn_2l_is_masked(const struct domain *d, evtchn_port_t port)
@@ -74,7 +77,8 @@ static bool evtchn_2l_is_masked(const st
     unsigned int max_ports = BITS_PER_EVTCHN_WORD(d) * BITS_PER_EVTCHN_WORD(d);
 
     ASSERT(port < max_ports);
-    return port >= max_ports || test_bit(port, &shared_info(d, evtchn_mask));
+    return (port >= max_ports ||
+            guest_test_bit(d, port, &shared_info(d, evtchn_mask)));
 }
 
 static void evtchn_2l_print_state(struct domain *d,
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -17,6 +17,8 @@
 #include <xen/mm.h>
 #include <xen/domain_page.h>
 
+#include <asm/guest_atomics.h>
+
 #include <public/event_channel.h>
 
 static inline event_word_t *evtchn_fifo_word_from_port(const struct domain *d,
@@ -51,7 +53,7 @@ static void evtchn_fifo_init(struct doma
      * on the wrong VCPU or with an unexpected priority.
      */
     word = evtchn_fifo_word_from_port(d, evtchn->port);
-    if ( word && test_bit(EVTCHN_FIFO_LINKED, word) )
+    if ( word && guest_test_bit(d, EVTCHN_FIFO_LINKED, word) )
         gdprintk(XENLOG_WARNING, "domain %d, port %d already on a queue\n",
                  d->domain_id, evtchn->port);
 }
@@ -116,7 +118,7 @@ static int try_set_link(event_word_t *wo
  * We block unmasking by the guest by marking the tail word as BUSY,
  * therefore, the cmpxchg() may fail at most 4 times.
  */
-static bool_t evtchn_fifo_set_link(const struct domain *d, event_word_t *word,
+static bool_t evtchn_fifo_set_link(struct domain *d, event_word_t *word,
                                    uint32_t link)
 {
     event_word_t w;
@@ -130,7 +132,7 @@ static bool_t evtchn_fifo_set_link(const
         return ret;
 
     /* Lock the word to prevent guest unmasking. */
-    set_bit(EVTCHN_FIFO_BUSY, word);
+    guest_set_bit(d, EVTCHN_FIFO_BUSY, word);
 
     w = read_atomic(word);
 
@@ -140,13 +142,13 @@ static bool_t evtchn_fifo_set_link(const
         if ( ret >= 0 )
         {
             if ( ret == 0 )
-                clear_bit(EVTCHN_FIFO_BUSY, word);
+                guest_clear_bit(d, EVTCHN_FIFO_BUSY, word);
             return ret;
         }
     }
     gdprintk(XENLOG_WARNING, "domain %d, port %d not linked\n",
              d->domain_id, link);
-    clear_bit(EVTCHN_FIFO_BUSY, word);
+    guest_clear_bit(d, EVTCHN_FIFO_BUSY, word);
     return 1;
 }
 
@@ -171,13 +173,13 @@ static void evtchn_fifo_set_pending(stru
         return;
     }
 
-    was_pending = test_and_set_bit(EVTCHN_FIFO_PENDING, word);
+    was_pending = guest_test_and_set_bit(d, EVTCHN_FIFO_PENDING, word);
 
     /*
      * Link the event if it unmasked and not already linked.
      */
-    if ( !test_bit(EVTCHN_FIFO_MASKED, word)
-         && !test_bit(EVTCHN_FIFO_LINKED, word) )
+    if ( !guest_test_bit(d, EVTCHN_FIFO_MASKED, word) &&
+         !guest_test_bit(d, EVTCHN_FIFO_LINKED, word) )
     {
         struct evtchn_fifo_queue *q, *old_q;
         event_word_t *tail_word;
@@ -206,7 +208,7 @@ static void evtchn_fifo_set_pending(stru
         if ( !old_q )
             goto done;
 
-        if ( test_and_set_bit(EVTCHN_FIFO_LINKED, word) )
+        if ( guest_test_and_set_bit(d, EVTCHN_FIFO_LINKED, word) )
         {
             spin_unlock_irqrestore(&old_q->lock, flags);
             goto done;
@@ -252,8 +254,8 @@ static void evtchn_fifo_set_pending(stru
         spin_unlock_irqrestore(&q->lock, flags);
 
         if ( !linked
-             && !test_and_set_bit(q->priority,
-                                  &v->evtchn_fifo->control_block->ready) )
+             && !guest_test_and_set_bit(d, q->priority,
+                                        &v->evtchn_fifo->control_block->ready) )
             vcpu_mark_events_pending(v);
     }
  done:
@@ -275,7 +277,7 @@ static void evtchn_fifo_clear_pending(st
      * No need to unlink as the guest will unlink and ignore
      * non-pending events.
      */
-    clear_bit(EVTCHN_FIFO_PENDING, word);
+    guest_clear_bit(d, EVTCHN_FIFO_PENDING, word);
 }
 
 static void evtchn_fifo_unmask(struct domain *d, struct evtchn *evtchn)
@@ -287,10 +289,10 @@ static void evtchn_fifo_unmask(struct do
     if ( unlikely(!word) )
         return;
 
-    clear_bit(EVTCHN_FIFO_MASKED, word);
+    guest_clear_bit(d, EVTCHN_FIFO_MASKED, word);
 
     /* Relink if pending. */
-    if ( test_bit(EVTCHN_FIFO_PENDING, word) )
+    if ( guest_test_bit(d, EVTCHN_FIFO_PENDING, word) )
         evtchn_fifo_set_pending(v, evtchn);
 }
 
@@ -298,21 +300,21 @@ static bool evtchn_fifo_is_pending(const
 {
     const event_word_t *word = evtchn_fifo_word_from_port(d, port);
 
-    return word && test_bit(EVTCHN_FIFO_PENDING, word);
+    return word && guest_test_bit(d, EVTCHN_FIFO_PENDING, word);
 }
 
 static bool_t evtchn_fifo_is_masked(const struct domain *d, evtchn_port_t port)
 {
     const event_word_t *word = evtchn_fifo_word_from_port(d, port);
 
-    return !word || test_bit(EVTCHN_FIFO_MASKED, word);
+    return !word || guest_test_bit(d, EVTCHN_FIFO_MASKED, word);
 }
 
 static bool_t evtchn_fifo_is_busy(const struct domain *d, evtchn_port_t port)
 {
     const event_word_t *word = evtchn_fifo_word_from_port(d, port);
 
-    return word && test_bit(EVTCHN_FIFO_LINKED, word);
+    return word && guest_test_bit(d, EVTCHN_FIFO_LINKED, word);
 }
 
 static int evtchn_fifo_set_priority(struct domain *d, struct evtchn *evtchn,
@@ -339,11 +341,11 @@ static void evtchn_fifo_print_state(stru
     word = evtchn_fifo_word_from_port(d, evtchn->port);
     if ( !word )
         printk("?     ");
-    else if ( test_bit(EVTCHN_FIFO_LINKED, word) )
-        printk("%c %-4u", test_bit(EVTCHN_FIFO_BUSY, word) ? 'B' : ' ',
+    else if ( guest_test_bit(d, EVTCHN_FIFO_LINKED, word) )
+        printk("%c %-4u", guest_test_bit(d, EVTCHN_FIFO_BUSY, word) ? 'B' : ' ',
                *word & EVTCHN_FIFO_LINK_MASK);
     else
-        printk("%c -   ", test_bit(EVTCHN_FIFO_BUSY, word) ? 'B' : ' ');
+        printk("%c -   ", guest_test_bit(d, EVTCHN_FIFO_BUSY, word) ? 'B' : ' ');
 }
 
 static const struct evtchn_port_ops evtchn_port_ops_fifo =
@@ -495,7 +497,7 @@ static void setup_ports(struct domain *d
 
         evtchn = evtchn_from_port(d, port);
 
-        if ( test_bit(port, &shared_info(d, evtchn_pending)) )
+        if ( guest_test_bit(d, port, &shared_info(d, evtchn_pending)) )
             evtchn->pending = 1;
 
         evtchn_fifo_set_priority(d, evtchn, EVTCHN_FIFO_PRIORITY_DEFAULT);
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -39,6 +39,7 @@
 #include <xen/vmap.h>
 #include <xsm/xsm.h>
 #include <asm/flushtlb.h>
+#include <asm/guest_atomics.h>
 
 /* Per-domain grant information. */
 struct grant_table {
@@ -646,6 +647,7 @@ static unsigned int nr_grant_entries(str
 }
 
 static int _set_status_v1(const grant_entry_header_t *shah,
+                          struct domain *rd,
                           struct active_grant_entry *act,
                           int readonly,
                           int mapflag,
@@ -701,8 +703,8 @@ static int _set_status_v1(const grant_en
                          "Attempt to write-pin a r/o grant entry\n");
         }
 
-        prev_scombo.word = cmpxchg((u32 *)shah,
-                                   scombo.word, new_scombo.word);
+        prev_scombo.word = guest_cmpxchg(rd, (u32 *)shah,
+                                         scombo.word, new_scombo.word);
         if ( likely(prev_scombo.word == scombo.word) )
             break;
 
@@ -719,6 +721,7 @@ done:
 
 static int _set_status_v2(const grant_entry_header_t *shah,
                           grant_status_t *status,
+                          struct domain *rd,
                           struct active_grant_entry *act,
                           int readonly,
                           int mapflag,
@@ -781,8 +784,8 @@ static int _set_status_v2(const grant_en
              (id != ldomid) ||
              (!readonly && (flags & GTF_readonly)) )
         {
-            gnttab_clear_flag(_GTF_writing, status);
-            gnttab_clear_flag(_GTF_reading, status);
+            gnttab_clear_flag(rd, _GTF_writing, status);
+            gnttab_clear_flag(rd, _GTF_reading, status);
             PIN_FAIL(done, GNTST_general_error,
                      "Unstable flags (%x) or dom (%d); expected d%d (r/w: %d)\n",
                      flags, id, ldomid, !readonly);
@@ -792,7 +795,7 @@ static int _set_status_v2(const grant_en
     {
         if ( unlikely(flags & GTF_readonly) )
         {
-            gnttab_clear_flag(_GTF_writing, status);
+            gnttab_clear_flag(rd, _GTF_writing, status);
             PIN_FAIL(done, GNTST_general_error,
                      "Unstable grant readonly flag\n");
         }
@@ -805,6 +808,7 @@ done:
 
 static int _set_status(const grant_entry_header_t *shah,
                        grant_status_t *status,
+                       struct domain *rd,
                        unsigned rgt_version,
                        struct active_grant_entry *act,
                        int readonly,
@@ -813,9 +817,9 @@ static int _set_status(const grant_entry
 {
 
     if ( rgt_version == 1 )
-        return _set_status_v1(shah, act, readonly, mapflag, ldomid);
+        return _set_status_v1(shah, rd, act, readonly, mapflag, ldomid);
     else
-        return _set_status_v2(shah, status, act, readonly, mapflag, ldomid);
+        return _set_status_v2(shah, status, rd, act, readonly, mapflag, ldomid);
 }
 
 static struct active_grant_entry *grant_map_exists(const struct domain *ld,
@@ -980,7 +984,7 @@ map_grant_ref(
          (!(op->flags & GNTMAP_readonly) &&
           !(act->pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask))) )
     {
-        if ( (rc = _set_status(shah, status, rgt->gt_version, act,
+        if ( (rc = _set_status(shah, status, rd, rgt->gt_version, act,
                                op->flags & GNTMAP_readonly, 1,
                                ld->domain_id) != GNTST_okay) )
             goto act_release_out;
@@ -1204,10 +1208,10 @@ map_grant_ref(
  unlock_out_clear:
     if ( !(op->flags & GNTMAP_readonly) &&
          !(act->pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask)) )
-        gnttab_clear_flag(_GTF_writing, status);
+        gnttab_clear_flag(rd, _GTF_writing, status);
 
     if ( !act->pin )
-        gnttab_clear_flag(_GTF_reading, status);
+        gnttab_clear_flag(rd, _GTF_reading, status);
 
  act_release_out:
     active_entry_release(act);
@@ -1477,10 +1481,10 @@ unmap_common_complete(struct gnttab_unma
 
     if ( ((act->pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask)) == 0) &&
          !(op->done & GNTMAP_readonly) )
-        gnttab_clear_flag(_GTF_writing, status);
+        gnttab_clear_flag(rd, _GTF_writing, status);
 
     if ( act->pin == 0 )
-        gnttab_clear_flag(_GTF_reading, status);
+        gnttab_clear_flag(rd, _GTF_reading, status);
 
     active_entry_release(act);
     grant_read_unlock(rgt);
@@ -2045,8 +2049,8 @@ gnttab_prepare_for_transfer(
         new_scombo = scombo;
         new_scombo.shorts.flags |= GTF_transfer_committed;
 
-        prev_scombo.word = cmpxchg((u32 *)&sha->flags,
-                                   scombo.word, new_scombo.word);
+        prev_scombo.word = guest_cmpxchg(rd, (u32 *)&sha->flags,
+                                         scombo.word, new_scombo.word);
         if ( likely(prev_scombo.word == scombo.word) )
             break;
 
@@ -2339,11 +2343,11 @@ release_grant_for_copy(
 
         act->pin -= GNTPIN_hstw_inc;
         if ( !(act->pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask)) )
-            gnttab_clear_flag(_GTF_writing, status);
+            gnttab_clear_flag(rd, _GTF_writing, status);
     }
 
     if ( !act->pin )
-        gnttab_clear_flag(_GTF_reading, status);
+        gnttab_clear_flag(rd, _GTF_reading, status);
 
     active_entry_release(act);
     grant_read_unlock(rgt);
@@ -2365,14 +2369,15 @@ release_grant_for_copy(
    under the domain's grant table lock. */
 /* Only safe on transitive grants.  Even then, note that we don't
    attempt to drop any pin on the referent grant. */
-static void fixup_status_for_copy_pin(const struct active_grant_entry *act,
+static void fixup_status_for_copy_pin(struct domain *rd,
+                                      const struct active_grant_entry *act,
                                       uint16_t *status)
 {
     if ( !(act->pin & (GNTPIN_hstw_mask | GNTPIN_devw_mask)) )
-        gnttab_clear_flag(_GTF_writing, status);
+        gnttab_clear_flag(rd, _GTF_writing, status);
 
     if ( !act->pin )
-        gnttab_clear_flag(_GTF_reading, status);
+        gnttab_clear_flag(rd, _GTF_reading, status);
 }
 
 /*
@@ -2434,7 +2439,7 @@ acquire_grant_for_copy(
     {
         if ( (!old_pin || (!readonly &&
                            !(old_pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask)))) &&
-             (rc = _set_status_v2(shah, status, act, readonly, 0,
+             (rc = _set_status_v2(shah, status, rd, act, readonly, 0,
                                   ldom)) != GNTST_okay )
             goto unlock_out;
 
@@ -2483,7 +2488,7 @@ acquire_grant_for_copy(
 
         if ( rc != GNTST_okay )
         {
-            fixup_status_for_copy_pin(act, status);
+            fixup_status_for_copy_pin(rd, act, status);
             rcu_unlock_domain(td);
             active_entry_release(act);
             grant_read_unlock(rgt);
@@ -2506,7 +2511,7 @@ acquire_grant_for_copy(
                           !act->is_sub_page)) )
         {
             release_grant_for_copy(td, trans_gref, readonly);
-            fixup_status_for_copy_pin(act, status);
+            fixup_status_for_copy_pin(rd, act, status);
             rcu_unlock_domain(td);
             active_entry_release(act);
             grant_read_unlock(rgt);
@@ -2535,7 +2540,7 @@ acquire_grant_for_copy(
     else if ( !old_pin ||
               (!readonly && !(old_pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask))) )
     {
-        if ( (rc = _set_status(shah, status, rgt->gt_version, act,
+        if ( (rc = _set_status(shah, status, rd, rgt->gt_version, act,
                                readonly, 0, ldom)) != GNTST_okay )
              goto unlock_out;
 
@@ -2623,10 +2628,10 @@ acquire_grant_for_copy(
  unlock_out_clear:
     if ( !(readonly) &&
          !(act->pin & (GNTPIN_hstw_mask | GNTPIN_devw_mask)) )
-        gnttab_clear_flag(_GTF_writing, status);
+        gnttab_clear_flag(rd, _GTF_writing, status);
 
     if ( !act->pin )
-        gnttab_clear_flag(_GTF_reading, status);
+        gnttab_clear_flag(rd, _GTF_reading, status);
 
  unlock_out:
     active_entry_release(act);
@@ -3661,11 +3666,11 @@ gnttab_release_mappings(
             }
 
             if ( (act->pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask)) == 0 )
-                gnttab_clear_flag(_GTF_writing, status);
+                gnttab_clear_flag(rd, _GTF_writing, status);
         }
 
         if ( act->pin == 0 )
-            gnttab_clear_flag(_GTF_reading, status);
+            gnttab_clear_flag(rd, _GTF_reading, status);
 
         active_entry_release(act);
         grant_read_unlock(rgt);
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -14,7 +14,7 @@ struct grant_table_arch {
     gfn_t *status_gfn;
 };
 
-void gnttab_clear_flag(unsigned long nr, uint16_t *addr);
+void gnttab_clear_flag(struct domain *d, unsigned long nr, uint16_t *addr);
 int create_grant_host_mapping(unsigned long gpaddr, mfn_t mfn,
                               unsigned int flags, unsigned int cache_flags);
 #define gnttab_host_mapping_get_page_type(ro, ld, rd) (0)
--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -64,7 +64,8 @@ static inline int replace_grant_host_map
 
 #define gnttab_mark_dirty(d, f) paging_mark_dirty((d), f)
 
-static inline void gnttab_clear_flag(unsigned int nr, uint16_t *st)
+static inline void gnttab_clear_flag(struct domain *d, unsigned int nr,
+                                     uint16_t *st)
 {
     /*
      * Note that this cannot be clear_bit(), as the access must be

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File 5d03a0c4-D-use-guest-atomics-helpers.patch of Package xen (Revision 1feb9822b6008e55083152e7093e909f)

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