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ivshmem: Require master to have ID zero

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Migration with ivshmem needs to be carefully orchestrated to work.
Exactly one peer (the "master") migrates to the destination, all other
peers need to unplug (and disconnect), migrate, plug back (and
reconnect).  This is sort of documented in qemu-doc.

If peers connect on the destination before migration completes, the
shared memory can get messed up.  This isn't documented anywhere.  Fix
that in qemu-doc.

To avoid messing up register IVPosition on migration, the server must
assign the same ID on source and destination.  ivshmem-spec.txt leaves
ID assignment unspecified, however.

Amend ivshmem-spec.txt to require the first client to receive ID zero.
The example ivshmem-server complies: it always assigns the first
unused ID.

For a bit of additional safety, enforce ID zero for the master.  This
does nothing when we're not using a server, because the ID is zero for
all peers then.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-40-git-send-email-armbru@redhat.com>
This commit is contained in:
Markus Armbruster 2016-03-15 19:34:54 +01:00
parent 13fd2cb689
commit 62a830b688
3 changed files with 13 additions and 0 deletions
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2
docs/specs/ivshmem-spec.txt
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@ -164,6 +164,8 @@ For each new client that connects to the server, the server
- sends interrupt setup messages to the new client (these contain file
descriptors for receiving interrupts).
The first client to connect to the server receives ID zero.
When a client disconnects from the server, the server sends disconnect
notifications to the other clients.

6
hw/misc/ivshmem.c
View File

@ -887,6 +887,12 @@ static void ivshmem_common_realize(PCIDevice *dev, Error **errp)
return;
}
if (s->master == ON_OFF_AUTO_ON && s->vm_id != 0) {
error_setg(errp,
"master must connect to the server before any peers");
return;
}
qemu_chr_add_handlers(s->server_chr, ivshmem_can_receive,
ivshmem_read, NULL, s);

5
qemu-doc.texi
View File

@ -1295,12 +1295,17 @@ When using the server, the guest will be assigned a VM ID (>=0) that allows gues
using the same server to communicate via interrupts. Guests can read their
VM ID from a device register (see ivshmem-spec.txt).
@subsubsection Migration with ivshmem
With device property @option{master=on}, the guest will copy the shared
memory on migration to the destination host. With @option{master=off},
the guest will not be able to migrate with the device attached. In the
latter case, the device should be detached and then reattached after
migration using the PCI hotplug support.
At most one of the devices sharing the same memory can be master. The
master must complete migration before you plug back the other devices.
@subsubsection ivshmem and hugepages
Instead of specifying the <shm size> using POSIX shm, you may specify