[PATCH qemu 2/2] dump: Only use the makedumpfile flattened format when necessary

[Stephen Brennan <stephen.s.brennan@oracle.com>]

The flattened format is used by makedumpfile only when it is outputting
a vmcore to a file which is not seekable. The flattened format functions
essentially as a set of instructions of the form "seek to the given
offset, then write the given bytes out".

The flattened format can be reconstructed using makedumpfile -R, or
makedumpfile-R.pl, but it is a slow process beacuse it requires copying
the entire vmcore. The flattened format can also be directly read by
crash, but still, it requires a lengthy reassembly phase.

To sum up, the flattened format is not an ideal one: it should only be
used on files which are actually not seekable. This is the exact
strategy which makedumpfile uses, as seen in the implementation of
"write_buffer()" in makedumpfile [1].

So, update the "dump-guest-memory" monitor command implementation so
that it will directly do the seeks and writes, in the same strategy as
makedumpfile. However, if the file is not seekable, then we can fall
back to the flattened format.

[1]: https://github.com/makedumpfile/makedumpfile/blob/f23bb943568188a2746dbf9b6692668f5a2ac3b6/makedumpfile.c#L5008-L5040

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
---
 dump/dump.c           | 30 +++++++++++++++++++++++++-----
 include/sysemu/dump.h |  1 +
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/dump/dump.c b/dump/dump.c
index 2708ddc135..384d275e39 100644
--- a/dump/dump.c
+++ b/dump/dump.c
@@ -813,6 +813,13 @@ static int write_start_flat_header(DumpState *s)
 {
     MakedumpfileHeader *mh;
     int ret = 0;
+    loff_t offset = lseek(s->fd, 0, SEEK_CUR);
+
+    /* If the file is seekable, don't output flattened header */
+    if (offset == 0) {
+        s->seekable = true;
+        return 0;
+    }
 
     QEMU_BUILD_BUG_ON(sizeof *mh > MAX_SIZE_MDF_HEADER);
     mh = g_malloc0(MAX_SIZE_MDF_HEADER);
@@ -837,6 +844,10 @@ static int write_end_flat_header(DumpState *s)
 {
     MakedumpfileDataHeader mdh;
 
+    if (s->seekable) {
+        return 0;
+    }
+
     mdh.offset = END_FLAG_FLAT_HEADER;
     mdh.buf_size = END_FLAG_FLAT_HEADER;
 
@@ -853,13 +864,21 @@ static int write_buffer(DumpState *s, off_t offset, const void *buf, size_t size
 {
     size_t written_size;
     MakedumpfileDataHeader mdh;
+    loff_t seek_loc;
 
-    mdh.offset = cpu_to_be64(offset);
-    mdh.buf_size = cpu_to_be64(size);
+    if (s->seekable) {
+        seek_loc = lseek(s->fd, offset, SEEK_SET);
+        if (seek_loc == (off_t) -1) {
+            return -1;
+        }
+    } else {
+        mdh.offset = cpu_to_be64(offset);
+        mdh.buf_size = cpu_to_be64(size);
 
-    written_size = qemu_write_full(s->fd, &mdh, sizeof(mdh));
-    if (written_size != sizeof(mdh)) {
-        return -1;
+        written_size = qemu_write_full(s->fd, &mdh, sizeof(mdh));
+        if (written_size != sizeof(mdh)) {
+            return -1;
+        }
     }
 
     written_size = qemu_write_full(s->fd, buf, size);
@@ -1786,6 +1805,7 @@ static void dump_init(DumpState *s, int fd, bool has_format,
     s->has_format = has_format;
     s->format = format;
     s->written_size = 0;
+    s->seekable = false;
 
     /* kdump-compressed is conflict with paging and filter */
     if (has_format && format != DUMP_GUEST_MEMORY_FORMAT_ELF) {
diff --git a/include/sysemu/dump.h b/include/sysemu/dump.h
index e27af8fb34..ab703c3a5e 100644
--- a/include/sysemu/dump.h
+++ b/include/sysemu/dump.h
@@ -157,6 +157,7 @@ typedef struct DumpState {
     MemoryMappingList list;
     bool resume;
     bool detached;
+    bool seekable;
     hwaddr memory_offset;
     int fd;
 
-- 
2.39.2