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[libvirt] [RFC][PATCH] Revision for message payload encoding error when adding a large mount of virtio disks

From: Chen Hanxiao <chenhanxiao cn fujitsu com>

When adding a large number of virtio storage devices to
virtual machine by using 'virsh edit' command, there is
a problem:
When we added more than 190 virtio disks by 'virsh edit'
command, we got a feedback as 'error: Unable to encode
message payload'. In virt-mananger, the same defect occurs
with about 180 virtio disks added.

PCI devices are limited by the virtualized system
architecture. Out of the 32 available PCI devices for a
guest, 4 are not removable. This means there are up to
28 free PCI slots available for additional devices per
guest. Each PCI device in a guest can have up to 8 functions.
One slot kept for network interface, we could add at most
216 (27*8) virtio disks.
The length of xml description with multifuction for one virtio
disk is about 290 characters; In virt-manger, an extra tag
'alias name' will come to the xml description, which would add
about 40 more characters.
So for one xml description, 330 characters would be enough.
A brand new virtual machine with one IDE bus disk needs about
1900 characters for xml description.

In src/remote/remote_protocol.x, there is a limitation for XDR
enoding length as REMOTE_STRING_MAX.
According to the analysis above, at least 73180(1900+216*330)
characters are needed for 216 virtio disks. In the view of
variable length in tag 'source file', so I think it would be
better to extend this limitation from 65536 to 80000.
 src/remote/remote_protocol.x |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/remote/remote_protocol.x b/src/remote/remote_protocol.x
index 2d57247..2c8dcbb 100644
--- a/src/remote/remote_protocol.x
+++ b/src/remote/remote_protocol.x
@@ -65,7 +65,7 @@
  * This is an arbitrary limit designed to stop the decoder from trying
  * to allocate unbounded amounts of memory when fed with a bad message.
-const REMOTE_STRING_MAX = 65536;
+const REMOTE_STRING_MAX = 80000;
 /* A long string, which may NOT be NULL. */
 typedef string remote_nonnull_string<REMOTE_STRING_MAX>;

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