[Xen-devel] xl save but leave domain paused

[Ian Murray <murrayie@xxxxxxxxxxx>]

All,

I have long desired an ability to execute a domain save and leave the 
domain in a paused state. This is so that I can initiate an LVM snapshot 
to go with the checkpoint file. I know I can achieve this via a 
non-checkpoint save and a restore, but it seems a bit silly to reload 
the domain's memory from disk and doubles the "suspension" time of the 
domain. So, ideally, I would like xl save -p that leaves the domain 
paused. I did put it in a feature request a long time ago, but it never 
made the cut.
To this end and as more of a starting point, I have written my own basic 
patch. While this appears to work, I see a (very small) opportunity for 
the domU to run for a short time between the libxl_domain_resume and 
libxl_domain_pause calls. This defeats the object as I am trying to 
maintain a disk snapshot that is exactly in synch with the save state.
Can anyone please offer some thoughts on how I can implement this 
properly. I have looked at the corresponding xc calls but meddling with 
those is way beyond my knowledge. Another way of looking at the problem 
would be able to perform an xl save on a paused domain, as this would 
achieve the same result.
Thanks for reading and thanks for any suggestions that are forthcoming. 
I am not a C guru and even less of a Xen dev guru, so please treat me 
somewhat like an idiot. :)
Thanks,

Ian.

(Against RELEASE-4.2.2)


diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
index 7780426..d0394df 100644
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -2976,7 +2976,7 @@ static void save_domain_core_writeconfig(int fd, 
const char *source,
             hdr.optional_data_len);
 }

-static int save_domain(const char *p, const char *filename, int checkpoint,
+static int save_domain(const char *p, const char *filename, int 
checkpoint, int leavepaused,
                 const char *override_config_file)
 {
     int fd;
@@ -3003,10 +3003,13 @@ static int save_domain(const char *p, const char 
*filename, int checkpoint,
     if (rc < 0)
         fprintf(stderr, "Failed to save domain, resuming domain\n");

-    if (checkpoint || rc < 0)
-        libxl_domain_resume(ctx, domid, 1, 0);
+    if (leavepaused || checkpoint || rc < 0) {
+       libxl_domain_resume(ctx, domid, 1, 0);
+        if (leavepaused && ! (rc < 0))
+            libxl_domain_pause(ctx, domid);
+    }
     else
-        libxl_domain_destroy(ctx, domid, 0);
+         libxl_domain_destroy(ctx, domid, 0);

     exit(rc < 0 ? 1 : 0);
 }
diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
index 7780426..d0394df 100644
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -2976,7 +2976,7 @@ static void save_domain_core_writeconfig(int fd, 
const char *source,
             hdr.optional_data_len);
 }

-static int save_domain(const char *p, const char *filename, int checkpoint,
+static int save_domain(const char *p, const char *filename, int 
checkpoint, int leavepaused,
                 const char *override_config_file)
 {
     int fd;
@@ -3003,10 +3003,13 @@ static int save_domain(const char *p, const char 
*filename, int checkpoint,
     if (rc < 0)
         fprintf(stderr, "Failed to save domain, resuming domain\n");

-    if (checkpoint || rc < 0)
-        libxl_domain_resume(ctx, domid, 1, 0);
+    if (leavepaused || checkpoint || rc < 0) {
+       libxl_domain_resume(ctx, domid, 1, 0);
+        if (leavepaused && ! (rc < 0))
+            libxl_domain_pause(ctx, domid);
+    }
     else
-        libxl_domain_destroy(ctx, domid, 0);
+         libxl_domain_destroy(ctx, domid, 0);

     exit(rc < 0 ? 1 : 0);
 }


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