[Xen-devel] [PATCH v5 4/6] libxl: support unmapping static shared memory areas during domain destruction

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

From: Zhongze Liu <blackskygg@xxxxxxxxx>

Author: Zhongze Liu <blackskygg@xxxxxxxxx>

Add libxl__sshm_del to unmap static shared memory areas mapped by
libxl__sshm_add during domain creation. The unmapping process is:

* For a master: decrease the refcount of the sshm region, if the refcount
  reaches 0, cleanup the whole sshm path.

* For a slave:
  1) unmap the shared pages, and cleanup related xs entries. If the
     system works normally, all the shared pages will be unmapped, so there
     won't be page leaks. In case of errors, the unmapping process will go
     on and unmap all the other pages that can be unmapped, so the other
     pages won't be leaked, either.
  2) Decrease the refcount of the sshm region, if the refcount reaches
     0, cleanup the whole sshm path.

This is for the proposal "Allow setting up shared memory areas between VMs
from xl config file" (see [1]).

[1] https://lists.xen.org/archives/html/xen-devel/2017-08/msg03242.html

Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>

Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxx

---
Changes in v5:
- fix typos
- add comments
- cannot move unmap before xenstore transaction because it needs to
  retrieve begin/size values from xenstore
---
 tools/libxl/libxl_domain.c   |   5 ++
 tools/libxl/libxl_internal.h |   2 +
 tools/libxl/libxl_sshm.c     | 107 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 114 insertions(+)

diff --git a/tools/libxl/libxl_domain.c b/tools/libxl/libxl_domain.c
index 533bcdf..053bbe2 100644
--- a/tools/libxl/libxl_domain.c
+++ b/tools/libxl/libxl_domain.c
@@ -1060,6 +1060,11 @@ void libxl__destroy_domid(libxl__egc *egc, 
libxl__destroy_domid_state *dis)
         goto out;
     }
 
+    rc = libxl__sshm_del(gc, domid);
+    if (rc) {
+        LOGD(ERROR, domid, "Deleting static shm failed.");
+    }
+
     if (libxl__device_pci_destroy_all(gc, domid) < 0)
         LOGD(ERROR, domid, "Pci shutdown failed");
     rc = xc_domain_pause(ctx->xch, domid);
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 076c30f..a2b81bf 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -4429,6 +4429,8 @@ static inline bool libxl__string_is_default(char **s)
 _hidden int libxl__sshm_add(libxl__gc *gc, uint32_t domid,
                             libxl_static_shm *sshm, int len);
 
+_hidden int libxl__sshm_del(libxl__gc *gc, uint32_t domid);
+
 _hidden int libxl__sshm_check_overlap(libxl__gc *gc, uint32_t domid,
                                       libxl_static_shm *sshms, int len);
 _hidden int libxl__sshm_setdefault(libxl__gc *gc, uint32_t domid,
diff --git a/tools/libxl/libxl_sshm.c b/tools/libxl/libxl_sshm.c
index f61b80c..9672056 100644
--- a/tools/libxl/libxl_sshm.c
+++ b/tools/libxl/libxl_sshm.c
@@ -94,6 +94,113 @@ int libxl__sshm_check_overlap(libxl__gc *gc, uint32_t domid,
     return 0;
 }
 
+/*
+ * Decrease the refcount of an sshm. When refcount reaches 0,
+ * clean up the whole sshm path.
+ * Xenstore operations are done within the same transaction.
+ */
+static void libxl__sshm_decref(libxl__gc *gc, xs_transaction_t xt,
+                               const char *sshm_path)
+{
+    int count;
+    const char *count_path, *count_string;
+
+    count_path = GCSPRINTF("%s/usercnt", sshm_path);
+    if (libxl__xs_read_checked(gc, xt, count_path, &count_string))
+        return;
+    count = atoi(count_string);
+
+    if (--count == 0) {
+        libxl__xs_path_cleanup(gc, xt, sshm_path);
+        return;
+    }
+
+    count_string = GCSPRINTF("%d", count);
+    libxl__xs_write_checked(gc, xt, count_path, count_string);
+
+    return;
+}
+
+static void libxl__sshm_do_unmap(libxl__gc *gc, uint32_t domid, const char *id,
+                                 uint64_t begin, uint64_t size)
+{
+    uint64_t end;
+    begin >>= XC_PAGE_SHIFT;
+    size >>= XC_PAGE_SHIFT;
+    end = begin + size;
+    for (; begin < end; ++begin) {
+        if (xc_domain_remove_from_physmap(CTX->xch, domid, begin)) {
+            SSHM_ERROR(domid, id,
+                       "unable to unmap shared page at 0x%"PRIx64".",
+                       begin);
+        }
+    }
+}
+
+static void libxl__sshm_del_slave(libxl__gc *gc, xs_transaction_t xt,
+                                  uint32_t domid, const char *id, bool isretry)
+{
+    const char *slave_path, *begin_str, *size_str;
+    uint64_t begin, size;
+
+    slave_path = GCSPRINTF("%s/slaves/%"PRIu32, SSHM_PATH(id), domid);
+
+    begin_str = libxl__xs_read(gc, xt, GCSPRINTF("%s/begin", slave_path));
+    size_str = libxl__xs_read(gc, xt, GCSPRINTF("%s/size", slave_path));
+    begin = strtoull(begin_str, NULL, 16);
+    size = strtoull(size_str, NULL, 16);
+
+    libxl__sshm_do_unmap(gc, domid, id, begin, size);
+    libxl__xs_path_cleanup(gc, xt, slave_path);
+}
+
+/* Delete static_shm entries in the xensotre. */
+int libxl__sshm_del(libxl__gc *gc,  uint32_t domid)
+{
+    int rc, i;
+    bool isretry;
+    xs_transaction_t xt = XBT_NULL;
+    const char *dom_path, *dom_sshm_path, *role;
+    char **sshm_ents;
+    unsigned int sshm_num;
+
+    dom_path = libxl__xs_get_dompath(gc, domid);
+    dom_sshm_path = GCSPRINTF("%s/static_shm", dom_path);
+
+    isretry = false;
+    for (;;) {
+        rc = libxl__xs_transaction_start(gc, &xt);
+        if (rc) goto out;
+
+        if (libxl__xs_read(gc, xt, dom_sshm_path)) {
+            sshm_ents = libxl__xs_directory(gc, xt, dom_sshm_path, &sshm_num);
+            if (!sshm_ents) continue;
+
+            for (i = 0; i < sshm_num; ++i) {
+                role = libxl__xs_read(gc, xt,
+                                      GCSPRINTF("%s/%s/role",
+                                                dom_sshm_path,
+                                                sshm_ents[i]));
+                assert(role);
+                if (!strncmp(role, "slave", 5))
+                    libxl__sshm_del_slave(gc, xt, domid, sshm_ents[i], 
isretry);
+
+                libxl__sshm_decref(gc, xt, SSHM_PATH(sshm_ents[i]));
+            }
+        }
+
+        rc = libxl__xs_transaction_commit(gc, &xt);
+        if (!rc) break;
+        if (rc < 0) goto out;
+        isretry = true;
+    }
+
+    rc = 0;
+out:
+    libxl__xs_transaction_abort(gc, &xt);
+    return rc;
+}
+
 /*   libxl__sshm_do_map -- map pages into slave's physmap
  *
  *   This functions maps
-- 
1.9.1


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