[PATCH v9 4/7] libxl: support unmapping static shared memory areas during domain destruction

[Stefano Stabellini <sstabellini@kernel.org>]

From: Zhongze Liu <blackskygg@gmail.com>

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

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

* For a borrower:
  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@gmail.com>
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
Cc: xen-devel@lists.xen.org

---
Changes in v9:
- rename master to owner and slave to borrower
- remove useless isretry parameter and variable
- code style
- add in-code comments
- call libxl__xs_directory directly
- check errno
- check for count_string == NULL
- fail for cases != owner and != borrower

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   |   8 +++
 tools/libxl/libxl_internal.h |   2 +
 tools/libxl/libxl_sshm.c     | 131 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 141 insertions(+)

diff --git a/tools/libxl/libxl_domain.c b/tools/libxl/libxl_domain.c
index 3377bba..f241edb 100644
--- a/tools/libxl/libxl_domain.c
+++ b/tools/libxl/libxl_domain.c
@@ -1060,6 +1060,14 @@ void libxl__destroy_domid(libxl__egc *egc, libxl__destroy_domid_state *dis)
         goto out;
     }
 
+    /*
+     * Only possible errors are unrecoverable xenstore transaction
+     * errors.
+     */
+    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 6f31a3d..e86d356 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -4442,6 +4442,8 @@ static inline const char *libxl__qemu_qmp_path(libxl__gc *gc, int domid)
 _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 d00981b..8395396 100644
--- a/tools/libxl/libxl_sshm.c
+++ b/tools/libxl/libxl_sshm.c
@@ -94,6 +94,137 @@ 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) ||
+        count_string == NULL)
+        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);
+        }
+    }
+}
+
+/* unmap static shared memory areas mapped by libxl__sshm_add */
+static void libxl__sshm_del_borrower(libxl__gc *gc, xs_transaction_t xt,
+                                     uint32_t domid, const char *id)
+{
+    const char *borrower_path, *begin_str, *size_str;
+    uint64_t begin, size;
+
+    borrower_path = GCSPRINTF("%s/borrowers/%"PRIu32, SSHM_PATH(id), domid);
+
+    begin_str = libxl__xs_read(gc, xt, GCSPRINTF("%s/begin", borrower_path));
+    size_str = libxl__xs_read(gc, xt, GCSPRINTF("%s/size", borrower_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, borrower_path);
+}
+
+/*
+ * Add libxl__sshm_del to unmap static shared memory areas mapped by
+ * libxl__sshm_add during domain creation. The unmapping process is:
+ *
+ * For a owner: decrease the refcount of the sshm region, if the refcount
+ * reaches 0, cleanup the whole sshm path.
+ *
+ * For a borrower:
+ * 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.
+ */
+int libxl__sshm_del(libxl__gc *gc,  uint32_t domid)
+{
+    int rc, i;
+    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);
+
+    for (;;) {
+        rc = libxl__xs_transaction_start(gc, &xt);
+        if (rc) goto out;
+
+        sshm_ents = libxl__xs_directory(gc, xt, dom_sshm_path, &sshm_num);
+        if (!sshm_ents) {
+            if (errno != ENOENT) {
+                LOGE(ERROR, "unable to get xenstore device listing %s",
+                     dom_sshm_path);
+                goto out;
+            }
+            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, "borrower", 8))
+                libxl__sshm_del_borrower(gc, xt, domid, sshm_ents[i]);
+            else if (strncmp(role, "owner", 5)) {
+                rc = ERROR_INVAL;
+                goto out;
+            }
+
+
+            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;
+    }
+
+    rc = 0;
+out:
+    libxl__xs_transaction_abort(gc, &xt);
+    return rc;
+}
+
 /*   libxl__sshm_do_map -- map pages into borrower's physmap
  *
  *   This functions maps
-- 
1.9.1


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