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docs: update QMP documents for OOB commands

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Update both the developer and spec for the new QMP OOB (Out-Of-Band)
command.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-2-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Peter Xu 2018-03-09 16:59:44 +08:00 • committed by Eric Blake
parent 99f2f54174
commit 378112b002
2 changed files with 82 additions and 12 deletions
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58
docs/devel/qapi-code-gen.txt
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@ -554,9 +554,12 @@ following example objects:
=== Commands ===
--- General Command Layout ---
Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
'*returns': TYPE-NAME, '*boxed': true,
'*gen': false, '*success-response': false }
'*gen': false, '*success-response': false,
'*allow-oob': true }
Commands are defined by using a dictionary containing several members,
where three members are most common. The 'command' member is a
@ -636,6 +639,49 @@ possible, the command expression should include the optional key
'success-response' with boolean value false. So far, only QGA makes
use of this member.
A command can be declared to support Out-Of-Band (OOB) execution. By
default, commands do not support OOB. To declare a command that
supports it, the schema includes an extra 'allow-oob' field. For
example:
{ 'command': 'migrate_recover',
'data': { 'uri': 'str' }, 'allow-oob': true }
To execute a command with out-of-band priority, the client specifies
the "control" field in the request, with "run-oob" set to
true. Example:
=> { "execute": "command-support-oob",
"arguments": { ... },
"control": { "run-oob": true } }
<= { "return": { } }
Without it, even the commands that support out-of-band execution will
still be run in-band.
Under normal QMP command execution, the following apply to each
command:
- They are executed in order,
- They run only in main thread of QEMU,
- They have the BQL taken during execution.
When a command is executed with OOB, the following changes occur:
- They can be completed before a pending in-band command,
- They run in a dedicated monitor thread,
- They do not take the BQL during execution.
OOB command handlers must satisfy the following conditions:
- It executes extremely fast,
- It does not take any lock, or, it can take very small locks if all
critical regions also follow the rules for OOB command handler code,
- It does not invoke system calls that may block,
- It does not access guest RAM that may block when userfaultfd is
enabled for postcopy live migration.
If in doubt, do not implement OOB execution support.
=== Events ===
@ -739,10 +785,12 @@ references by name.
QAPI schema definitions not reachable that way are omitted.
The SchemaInfo for a command has meta-type "command", and variant
members "arg-type" and "ret-type". On the wire, the "arguments"
member of a client's "execute" command must conform to the object type
named by "arg-type". The "return" member that the server passes in a
success response conforms to the type named by "ret-type".
members "arg-type", "ret-type" and "allow-oob". On the wire, the
"arguments" member of a client's "execute" command must conform to the
object type named by "arg-type". The "return" member that the server
passes in a success response conforms to the type named by
"ret-type". When "allow-oob" is set, it means the command supports
out-of-band execution.
If the command takes no arguments, "arg-type" names an object type
without members. Likewise, if the command returns nothing, "ret-type"