Patchwork [RFC,14/16] Start some BER format docs

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Submitter Dave Gilbert
Date March 25, 2014, 8:17 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/333727/
State New
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Dave Gilbert - March 25, 2014, 8:17 p.m.
From: "Dr. David Alan Gilbert" <>

Add information on the QEMUFile/Visitor/VMState split
Describe how to verify a BER stream against the schema

Signed-off-by: Dr. David Alan Gilbert <>
 docs/migration.txt | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)


diff --git a/docs/migration.txt b/docs/migration.txt
index 0e0a1d4..894abf3 100644
--- a/docs/migration.txt
+++ b/docs/migration.txt
@@ -48,8 +48,17 @@  This is used for RAM and block devices.  It is not yet ported to vmstate.
 === What is the common infrastructure ===
-QEMU uses a QEMUFile abstraction to be able to do migration.  Any type
-of migration that wants to use QEMU infrastructure has to create a
+Migration consists of a number of layers that handle different parts of the
+    QEMUFile - Buffering/reading/writing data without worrying about it's
+               contents. Versions exist for file I/O, sockets, RDMA and
+               to put data into a QEMU Block device.
+    Visitor  - Visitors format data onto the QEMUFile. A binary format and
+               a BER format currently exist.
+    VMState  - Represent the structure of the data for a given device with
+               support for versioning.
+Any type of migration that wants to use QEMU infrastructure has to create a
 QEMUFile with:
 QEMUFile *qemu_fopen_ops(void *opaque,
@@ -296,3 +305,24 @@  save/send this state when we are in the middle of a pio operation
 (that is what ide_drive_pio_state_needed() checks).  If DRQ_STAT is
 not enabled, the values on that fields are garbage and don't need to
 be sent.
+ASN.1/BER format migration
+In BER format migration the migration stream is described by an ASN.1 schema
+(that's defined in docs/specs/migration.schema).
+The 'asn1c' program can be used to validate a migration stream against the schema
+ - although it'll probably use a lot of memory doing it - so it's best to do it
+with a small RAM image.
+To validate an image against the schema:
+  a) Make a temporary directory and cd into it
+  b) asn1c -fcompound-names /path/to/migration.schema
+  c) make -f CFLAGS+="-DPDU=QemuFile -DEMIT_ASN_DEBUG=1 -I."
+  d) ./progname -iber log.ber -d 2> debug > decoding
+  If it's happy the 'decoding' output should be an XML like dump of the input file;
+if it's not happy, the debug file might help you find out why (Search backwards
+from the end for the first 'Freeing' to find the point at which it detected something