[v10,05/31] tools/libx{l, c}: add back channel to libxc

Commit Message

Wen Congyang Feb. 22, 2016, 2:52 a.m. UTC

In COLO mode, both VMs are running, and are considered in sync if the
visible network traffic is identical.  After some time, they fall out of
sync.

At this point, the two VMs have definitely diverged.  Lets call the
primary dirty bitmap set A, while the secondary dirty bitmap set B.

Sets A and B are different.

Under normal migration, the page data for set A will be sent from the
primary to the secondary.

However, the set difference B - A (the one in B but not in A, lets
call this C) is out-of-date on the secondary (with respect to the
primary) and will not be sent by the primary (to secondary), as it
was not memory dirtied by the primary. The secondary needs C page data
to reconstruct an exact copy of the primary at the checkpoint.

The secondary cannot calculate C as it doesn't know A.  Instead, the
secondary must send B to the primary, at which point the primary
calculates the union of A and B (lets call this D) which is all the
pages dirtied by both the primary and the secondary, and sends all page
data covered by D.

In the general case, D is a superset of both A and B.  Without the
backchannel dirty bitmap, a COLO checkpoint can't reconstruct a valid
copy of the primary.

We transfer the dirty bitmap on libxc side, so we need to introduce back
channel to libxc.

Note: it is different from the paper. We change the original design to
the current one, according to our following concerns:
1. The original design needs extra memory on Secondary host. When there's
   multiple backups on one host, the memory cost is high.
2. The memory cache code will be another 1k+, it will make the review
   more time consuming.

Note: the back channel will be used in the patch
 libxc/restore: send dirty pfn list to primary when checkpoint under COLO
to send dirty pfn list from secondary to primary. The patch is posted in
another series.

Signed-off-by: Yang Hongyang <[e-mail]>
Signed-off-by: Andrew Cooper <[e-mail]>
CC: Ian Campbell <[e-mail]>
CC: Ian Jackson <[e-mail]>
CC: Wei Liu <[e-mail]>
Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>
---
 tools/libxc/include/xenguest.h   |  4 ++--
 tools/libxc/xc_nomigrate.c       |  4 ++--
 tools/libxc/xc_sr_restore.c      |  2 +-
 tools/libxc/xc_sr_save.c         |  2 +-
 tools/libxl/libxl_save_callout.c | 21 +++++++++++++++------
 tools/libxl/libxl_save_helper.c  |  8 ++++++--
 6 files changed, 27 insertions(+), 14 deletions(-)

Comments

Wei Liu Feb. 25, 2016, 3:54 p.m. UTC | #1

On Mon, Feb 22, 2016 at 10:52:09AM +0800, Wen Congyang wrote:
> In COLO mode, both VMs are running, and are considered in sync if the
> visible network traffic is identical.  After some time, they fall out of
> sync.
> 
> At this point, the two VMs have definitely diverged.  Lets call the
> primary dirty bitmap set A, while the secondary dirty bitmap set B.
> 
> Sets A and B are different.
> 
> Under normal migration, the page data for set A will be sent from the
> primary to the secondary.
> 
> However, the set difference B - A (the one in B but not in A, lets
> call this C) is out-of-date on the secondary (with respect to the
> primary) and will not be sent by the primary (to secondary), as it
> was not memory dirtied by the primary. The secondary needs C page data
> to reconstruct an exact copy of the primary at the checkpoint.
> 
> The secondary cannot calculate C as it doesn't know A.  Instead, the
> secondary must send B to the primary, at which point the primary
> calculates the union of A and B (lets call this D) which is all the
> pages dirtied by both the primary and the secondary, and sends all page
> data covered by D.
> 
> In the general case, D is a superset of both A and B.  Without the
> backchannel dirty bitmap, a COLO checkpoint can't reconstruct a valid
> copy of the primary.
> 
> We transfer the dirty bitmap on libxc side, so we need to introduce back
> channel to libxc.
> 
> Note: it is different from the paper. We change the original design to
> the current one, according to our following concerns:
> 1. The original design needs extra memory on Secondary host. When there's
>    multiple backups on one host, the memory cost is high.
> 2. The memory cache code will be another 1k+, it will make the review
>    more time consuming.
> 
> Note: the back channel will be used in the patch
>  libxc/restore: send dirty pfn list to primary when checkpoint under COLO
> to send dirty pfn list from secondary to primary. The patch is posted in
> another series.

I think you need to update the commit message a bit. I take the liberty
of writing one for you:

This patch merely adds new parameters to various prototypes and
functions. The new parameters are used in later patch called
"libxc/restore: send dirty pfn list to primary when checkpoint under
COLO".

I would normally ask the submitter to merge patch like this to the
actual patch that uses the parameter. It you can do that, that would be
great. But this patch series is already quite large, if this involves a
lot of conflict resolution on your side, just leave it as-is.

Either way, please confirm your intention before doing actual work.

Bottom line: patch like this needs to compile and not introduce any
regression. I think that's the case here.

Wei.

Patch

diff --git a/tools/libxc/include/xenguest.h b/tools/libxc/include/xenguest.h
index affc42b..8040ac7 100644
--- a/tools/libxc/include/xenguest.h
+++ b/tools/libxc/include/xenguest.h
@@ -88,7 +88,7 @@  struct save_callbacks {
 int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom, uint32_t max_iters,
                    uint32_t max_factor, uint32_t flags /* XCFLAGS_xxx */,
                    struct save_callbacks* callbacks, int hvm,
-                   int checkpointed_stream);
+                   int checkpointed_stream, int recv_fd);
 
 /* callbacks provided by xc_domain_restore */
 struct restore_callbacks {
@@ -127,7 +127,7 @@  int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
                       unsigned long *console_mfn, domid_t console_domid,
                       unsigned int hvm, unsigned int pae, int superpages,
                       int checkpointed_stream,
-                      struct restore_callbacks *callbacks);
+                      struct restore_callbacks *callbacks, int send_back_fd);
 
 /**
  * This function will create a domain for a paravirtualized Linux
diff --git a/tools/libxc/xc_nomigrate.c b/tools/libxc/xc_nomigrate.c
index c9124df..3634743 100644
--- a/tools/libxc/xc_nomigrate.c
+++ b/tools/libxc/xc_nomigrate.c
@@ -23,7 +23,7 @@ 
 int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom, uint32_t max_iters,
                    uint32_t max_factor, uint32_t flags,
                    struct save_callbacks* callbacks, int hvm,
-                   int checkpointed_stream)
+                   int checkpointed_stream, int recv_fd)
 {
     errno = ENOSYS;
     return -1;
@@ -35,7 +35,7 @@  int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
                       unsigned long *console_mfn, domid_t console_domid,
                       unsigned int hvm, unsigned int pae, int superpages,
                       int checkpointed_stream,
-                      struct restore_callbacks *callbacks)
+                      struct restore_callbacks *callbacks, int send_back_fd)
 {
     errno = ENOSYS;
     return -1;
diff --git a/tools/libxc/xc_sr_restore.c b/tools/libxc/xc_sr_restore.c
index d4d33fd..e24560e 100644
--- a/tools/libxc/xc_sr_restore.c
+++ b/tools/libxc/xc_sr_restore.c
@@ -726,7 +726,7 @@  int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
                       unsigned long *console_gfn, domid_t console_domid,
                       unsigned int hvm, unsigned int pae, int superpages,
                       int checkpointed_stream,
-                      struct restore_callbacks *callbacks)
+                      struct restore_callbacks *callbacks, int send_back_fd)
 {
     struct xc_sr_context ctx =
         {
diff --git a/tools/libxc/xc_sr_save.c b/tools/libxc/xc_sr_save.c
index e258b7c..fe210cc 100644
--- a/tools/libxc/xc_sr_save.c
+++ b/tools/libxc/xc_sr_save.c
@@ -830,7 +830,7 @@  static int save(struct xc_sr_context *ctx, uint16_t guest_type)
 int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom,
                    uint32_t max_iters, uint32_t max_factor, uint32_t flags,
                    struct save_callbacks* callbacks, int hvm,
-                   int checkpointed_stream)
+                   int checkpointed_stream, int recv_fd)
 {
     struct xc_sr_context ctx =
         {
diff --git a/tools/libxl/libxl_save_callout.c b/tools/libxl/libxl_save_callout.c
index a885504..0d6949a 100644
--- a/tools/libxl/libxl_save_callout.c
+++ b/tools/libxl/libxl_save_callout.c
@@ -27,7 +27,7 @@ 
  */
 static void run_helper(libxl__egc *egc, libxl__save_helper_state *shs,
                        const char *mode_arg,
-                       int stream_fd,
+                       int stream_fd, int back_channel_fd,
                        const int *preserve_fds, int num_preserve_fds,
                        const unsigned long *argnums, int num_argnums);
 
@@ -50,6 +50,7 @@  void libxl__xc_domain_restore(libxl__egc *egc, libxl__domain_create_state *dcs,
     /* Convenience aliases */
     const uint32_t domid = dcs->guest_domid;
     const int restore_fd = dcs->libxc_fd;
+    const int send_back_fd = dcs->send_back_fd;
     libxl__domain_build_state *const state = &dcs->build_state;
 
     unsigned cbflags =
@@ -71,7 +72,7 @@  void libxl__xc_domain_restore(libxl__egc *egc, libxl__domain_create_state *dcs,
     shs->caller_state = dcs;
     shs->need_results = 1;
 
-    run_helper(egc, shs, "--restore-domain", restore_fd, 0, 0,
+    run_helper(egc, shs, "--restore-domain", restore_fd, send_back_fd, 0, 0,
                argnums, ARRAY_SIZE(argnums));
 }
 
@@ -95,7 +96,7 @@  void libxl__xc_domain_save(libxl__egc *egc, libxl__domain_save_state *dss,
     shs->caller_state = dss;
     shs->need_results = 0;
 
-    run_helper(egc, shs, "--save-domain", dss->fd,
+    run_helper(egc, shs, "--save-domain", dss->fd, dss->recv_fd,
                NULL, 0,
                argnums, ARRAY_SIZE(argnums));
     return;
@@ -139,12 +140,14 @@  static int dup_fd_helper(libxl__gc *gc, int fd, const char *what)
  * 1) Path to libxl-save-helper.
  * 2) --[restore|save]-domain.
  * 3) stream file descriptor.
+ * 4) back channel file descriptor.
  * n) save/restore specific parameters.
- * 4) A \0 at the end.
+ * 5) A \0 at the end.
  */
-#define HELPER_NR_ARGS 4
+#define HELPER_NR_ARGS 5
 static void run_helper(libxl__egc *egc, libxl__save_helper_state *shs,
-                       const char *mode_arg, int stream_fd,
+                       const char *mode_arg,
+                       int stream_fd, int back_channel_fd,
                        const int *preserve_fds, int num_preserve_fds,
                        const unsigned long *argnums, int num_argnums)
 {
@@ -177,6 +180,7 @@  static void run_helper(libxl__egc *egc, libxl__save_helper_state *shs,
     *arg++ = getenv("LIBXL_SAVE_HELPER") ?: LIBEXEC_BIN "/" "libxl-save-helper";
     *arg++ = mode_arg;
     const char **stream_fd_arg = arg++;
+    const char **back_channel_fd_arg = arg++;
     for (i=0; i<num_argnums; i++)
         *arg++ = GCSPRINTF("%lu", argnums[i]);
     *arg++ = 0;
@@ -204,6 +208,11 @@  static void run_helper(libxl__egc *egc, libxl__save_helper_state *shs,
         stream_fd = dup_fd_helper(gc, stream_fd, "migration stream fd");
         *stream_fd_arg = GCSPRINTF("%d", stream_fd);
 
+        if (back_channel_fd >= 0)
+            back_channel_fd = dup_fd_helper(gc, back_channel_fd,
+                                            "migration back channel fd");
+        *back_channel_fd_arg = GCSPRINTF("%d", back_channel_fd);
+
         for (i=0; i<num_preserve_fds; i++)
             if (preserve_fds[i] >= 0) {
                 assert(preserve_fds[i] > 2);
diff --git a/tools/libxl/libxl_save_helper.c b/tools/libxl/libxl_save_helper.c
index 6bdcf13..2ff4c6e 100644
--- a/tools/libxl/libxl_save_helper.c
+++ b/tools/libxl/libxl_save_helper.c
@@ -238,6 +238,7 @@  static struct restore_callbacks helper_restore_callbacks;
 int main(int argc, char **argv)
 {
     int r;
+    int send_back_fd, recv_fd;
 
 #define NEXTARG (++argv, assert(*argv), *argv)
 
@@ -247,6 +248,7 @@  int main(int argc, char **argv)
     if (!strcmp(mode,"--save-domain")) {
 
         io_fd =                    atoi(NEXTARG);
+        recv_fd =                  atoi(NEXTARG);
         uint32_t dom =             strtoul(NEXTARG,0,10);
         uint32_t max_iters =       strtoul(NEXTARG,0,10);
         uint32_t max_factor =      strtoul(NEXTARG,0,10);
@@ -262,12 +264,14 @@  int main(int argc, char **argv)
         setup_signals(save_signal_handler);
 
         r = xc_domain_save(xch, io_fd, dom, max_iters, max_factor, flags,
-                           &helper_save_callbacks, hvm, checkpointed_stream);
+                           &helper_save_callbacks, hvm, checkpointed_stream,
+                           recv_fd);
         complete(r);
 
     } else if (!strcmp(mode,"--restore-domain")) {
 
         io_fd =                    atoi(NEXTARG);
+        send_back_fd =             atoi(NEXTARG);
         uint32_t dom =             strtoul(NEXTARG,0,10);
         unsigned store_evtchn =    strtoul(NEXTARG,0,10);
         domid_t store_domid =      strtoul(NEXTARG,0,10);
@@ -292,7 +296,7 @@  int main(int argc, char **argv)
                               store_domid, console_evtchn, &console_mfn,
                               console_domid, hvm, pae, superpages,
                               checkpointed,
-                              &helper_restore_callbacks);
+                              &helper_restore_callbacks, send_back_fd);
         helper_stub_restore_results(store_mfn,console_mfn,0);
         complete(r);