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Re: [Qemu-devel] Re: RFC: blockdev_add & friends, brief rationale, QMP d


From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: RFC: blockdev_add & friends, brief rationale, QMP docs
Date: Fri, 28 May 2010 14:17:07 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.5) Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 05/28/2010 02:13 PM, Kevin Wolf wrote:
Am 28.05.2010 20:21, schrieb Markus Armbruster:
I'd like to give posting documentation of new QMP commands for review
before posting code a try.  But first let me explain briefly why we need
new commands.

We want a clean separation between host part (blockdev_add) and guest
part (device_add).  Existing -drive and drive_add don't provide that;
they were designed to specify both parts together.  Moreover, drive_add
is limited to adding virtio drives (with pci_add's help) and SCSI
drives.

Support for defining just a host part for use with -device and was
grafted onto -drive (if=none), but it's a mess.  Some parts are
redundant, other parts are broken.

For instance, unit, bus, index, addr are redundant: -device does not use
them, it provides its own parameters to specify bus and bus-specific
address.

The checks whether rerror, werror, readonly, cyls, heads, secs are sane
for a particular guest driver are broken.  The checks are in the -drive
code, which used to know the guest driver, but doesn't with if=none.

Additionally, removable media are flawed.  Many parameters set with
-drive silently revert to defaults on media change.

My proposed solution is a new option -blockdev and monitor command
blockdev_add.  These specify only the host drive.  Guest drive
properties are left to -device / device_add.  We keep -drive for
backwards compatibility and command line convenience.  Except we get rid
of if=none (may need a grace period).

New monitor command blockdev_del works regardless of how the host block
device was created.

New monitor command blockdev_change provides full control over the host
part, unlike the existing change command.

Summary of the host / guest split:

-drive options                      host or guest?
bus, unit, if, index, addr          guest, already covered by qdev
cyls, heads, secs, trans            guest, new qdev properties
                                       (but defaults depend on image)
media                               guest
snapshot, file, cache, aio, format  host, blockdev_add options
rerror, werror                      host, guest drivers will reject
                                       values they don't support
serial                              guest, new qdev properties
readonly                            both host&  guest, qdev will refuse
                                       to connect readonly host to read/
                                       write guest

QMP command docs:

blockdev_add
------------

Add host block device.

Arguments:

- "id": the host block device's ID, must be unique (json-string)
- "file": the disk image file to use (json-string, optional)
- "format": disk format (json-string, optional)
     - Possible values: "raw", "qcow2", ...
- "aio": host AIO (json-string, optional)
     - Possible values: "threads" (default), "native"
- "cache": host cache usage (json-string, optional)
     - Possible values: "writethrough" (default), "writeback", "unsafe",
                        "none"
- "readonly": open image read-only (json-bool, optional, default false)
- "rerror": what to do on read error (json-string, optional)
     - Possible values: "report" (default), "ignore", "stop"
- "werror": what to do on write error (json-string, optional)
     - Possible values: "enospc" (default), "report", "ignore", "stop"
- "snapshot": enable snapshot (json-bool, optional, default false)

Example:

->  { "execute": "blockdev_add",
              "arguments": { "format": "raw", "id": "blk1",
                             "file": "fedora.img" } }
<- { "return": {} }

Notes:

(1) If argument "file" is missing, all other optional arguments must be
     missing as well.  This defines a block device with no media
     inserted.

(2) It's possible to list supported disk formats by running QEMU with
     arguments "-blockdev_add \?".
-blockdev without _add you probably mean, if it's a command line option.

Maybe one more thing to consider is encrypted images. With "change" in
the user monitor you're automatically prompted for the password.

I'm not sure how this is supposed to work with QMP. From the
do_change_block code it looks as if you'd get an error and had to send a
block_set_passwd as a response to that. In the meantime the image would
be kind of half-open? What do devices do with it until the key is provided?

If a password is needed, we should throw an error and let the QMP client set the password and try again.

Regards,

Anthony Liguori

Would it make s1ense to add a password field to blockdev_add/change to
avoid this?

Kevin





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