Re: [Qemu-devel] [RFC PATCH 11/34] hyperv: add synic message delivery

[Paolo Bonzini]

On 06/02/2018 21:30, Roman Kagan wrote:
> +
> +    HvSintMsgCb msg_cb;
> +    void *msg_cb_data;
> +    struct hyperv_message *msg;
> +    /*
> +     * the state of the message staged in .msg:
> +     * 0        - the staging area is not in use (after init or message
> +     *            successfully delivered to guest)
> +     * -EBUSY   - the staging area is being used in vcpu thread
> +     * -EAGAIN  - delivery attempt failed due to slot being busy, retry
> +     * -EXXXX   - error
> +     */
> +    int msg_status;
> +

Access to these fields needs to be protected by a mutex (the refcount is
okay because it is only released in the bottom half).  Please also add
comments regarding which fields are read-only, which are accessed under
BQL, etc.

Also, msg_status's handling of EBUSY/EAGAIN is a bit strange, like:

+    if (ret == -EBUSY) {
+        return -EAGAIN;
+    }
+    if (ret) {
+        return ret;
+    }

I wonder if it would be better to change -EBUSY to 1, or to split
msg_status into two fields, such as msg_status (filled by the vCPU
thread) and msg_state which can be HYPERV_MSG_STATE_{FREE,BUSY,POSTED}.
msg_status is only valid if the state is POSTED, and sint_msg_bh then
moves the state back to FREE.

Thanks,

Paolo