diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/qapi-code-gen.txt | 17 |
1 files changed, 11 insertions, 6 deletions
diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt index 88a70e4d45..94a7e8f4d0 100644 --- a/docs/devel/qapi-code-gen.txt +++ b/docs/devel/qapi-code-gen.txt @@ -666,22 +666,27 @@ command: - They are executed in order, - They run only in main thread of QEMU, -- They have the BQL taken during execution. +- They run with the BQL held. 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. +- They run with the BQL not held. 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 terminates quickly, - 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. + enabled for postcopy live migration, +- It takes only "fast" locks, i.e. all critical sections protected by + any lock it takes also satisfy the conditions for OOB command + handler code. + +The restrictions on locking limit access to shared state. Such access +requires synchronization, but OOB commands can't take the BQL or any +other "slow" lock. If in doubt, do not implement OOB execution support. |