[v6,1/5] memory: Reference as->current_map directly in memory commit

Commit Message

Chuang Xu March 3, 2023, 10:39 a.m. UTC

From: Peter Xu <[e-mail]>

Calling RCU variance of address_space_get|to_flatview() during memory
commit (flatview updates, triggering memory listeners, or updating
ioeventfds, etc.) is not 100% accurate, because commit() requires BQL
rather than RCU read lock, so the context exclusively owns current_map and
can be directly referenced.

Neither does it need a refcount to current_map because it cannot be freed
from under the caller.

Add address_space_get_flatview_raw() for the case where the context holds
BQL rather than RCU read lock and use it across the core memory updates,
Drop the extra refcounts on FlatView*.

Signed-off-by: Peter Xu <[e-mail]>
---
 softmmu/memory.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

Patch

diff --git a/softmmu/memory.c b/softmmu/memory.c
index 9d64efca26..213496802b 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -61,6 +61,13 @@  struct AddrRange {
     Int128 size;
 };
 
+/* Called with BQL held */
+static inline FlatView *address_space_to_flatview_raw(AddressSpace *as)
+{
+    assert(qemu_mutex_iothread_locked());
+    return as->current_map;
+}
+
 static AddrRange addrrange_make(Int128 start, Int128 size)
 {
     return (AddrRange) { start, size };
@@ -155,7 +162,7 @@  enum ListenerDirection { Forward, Reverse };
 #define MEMORY_LISTENER_UPDATE_REGION(fr, as, dir, callback, _args...)  \
     do {                                                                \
         MemoryRegionSection mrs = section_from_flat_range(fr,           \
-                address_space_to_flatview(as));                         \
+                address_space_to_flatview_raw(as));                     \
         MEMORY_LISTENER_CALL(as, callback, dir, &mrs, ##_args);         \
     } while(0)
 
@@ -753,6 +760,7 @@  static FlatView *generate_memory_topology(MemoryRegion *mr)
 }
 
 static void address_space_add_del_ioeventfds(AddressSpace *as,
+                                             FlatView *view,
                                              MemoryRegionIoeventfd *fds_new,
                                              unsigned fds_new_nb,
                                              MemoryRegionIoeventfd *fds_old,
@@ -774,7 +782,7 @@  static void address_space_add_del_ioeventfds(AddressSpace *as,
                                                   &fds_new[inew]))) {
             fd = &fds_old[iold];
             section = (MemoryRegionSection) {
-                .fv = address_space_to_flatview(as),
+                .fv = view,
                 .offset_within_address_space = int128_get64(fd->addr.start),
                 .size = fd->addr.size,
             };
@@ -787,7 +795,7 @@  static void address_space_add_del_ioeventfds(AddressSpace *as,
                                                          &fds_old[iold]))) {
             fd = &fds_new[inew];
             section = (MemoryRegionSection) {
-                .fv = address_space_to_flatview(as),
+                .fv = view,
                 .offset_within_address_space = int128_get64(fd->addr.start),
                 .size = fd->addr.size,
             };
@@ -833,7 +841,7 @@  static void address_space_update_ioeventfds(AddressSpace *as)
     ioeventfd_max = QEMU_ALIGN_UP(as->ioeventfd_nb, 4);
     ioeventfds = g_new(MemoryRegionIoeventfd, ioeventfd_max);
 
-    view = address_space_get_flatview(as);
+    view = address_space_to_flatview_raw(as);
     FOR_EACH_FLAT_RANGE(fr, view) {
         for (i = 0; i < fr->mr->ioeventfd_nb; ++i) {
             tmp = addrrange_shift(fr->mr->ioeventfds[i].addr,
@@ -852,13 +860,12 @@  static void address_space_update_ioeventfds(AddressSpace *as)
         }
     }
 
-    address_space_add_del_ioeventfds(as, ioeventfds, ioeventfd_nb,
+    address_space_add_del_ioeventfds(as, view, ioeventfds, ioeventfd_nb,
                                      as->ioeventfds, as->ioeventfd_nb);
 
     g_free(as->ioeventfds);
     as->ioeventfds = ioeventfds;
     as->ioeventfd_nb = ioeventfd_nb;
-    flatview_unref(view);
 }
 
 /*
@@ -1026,7 +1033,7 @@  static void flatviews_reset(void)
 
 static void address_space_set_flatview(AddressSpace *as)
 {
-    FlatView *old_view = address_space_to_flatview(as);
+    FlatView *old_view = address_space_to_flatview_raw(as);
     MemoryRegion *physmr = memory_region_get_flatview_root(as->root);
     FlatView *new_view = g_hash_table_lookup(flat_views, physmr);