Re: [PATCH 13/14] migration/multifd: Move header prepare/fill into send_prepare()

[Peter Xu <peterx@redhat.com>]

On Wed, Jan 31, 2024 at 06:31:10PM +0800, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> This patch redefines the interfacing of ->send_prepare().  It further
> simplifies multifd_send_thread() especially on zero copy.
> 
> Now with the new interface, we require the hook to do all the work for
> preparing the IOVs to send.  After it's completed, the IOVs should be ready
> to be dumped into the specific multifd QIOChannel later.
> 
> So now the API looks like:
> 
>   p->pages ----------->  send_prepare() -------------> IOVs
> 
> This also prepares for the case where the input can be extended to even not
> any p->pages.  But that's for later.
> 
> This patch will achieve similar goal of what Fabiano used to propose here:
> 
> https://lore.kernel.org/r/20240126221943.26628-1-farosas@suse.de
> 
> However the send() interface may not be necessary.  I'm boldly attaching a
> "Co-developed-by" for Fabiano.
> 
> Co-developed-by: Fabiano Rosas <farosas@suse.de>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Just a heads-up: I plan to squash something like below also into it.
That's mostly Fabiano's:

https://lore.kernel.org/r/20240126221943.26628-6-farosas@suse.de

But instead of overwritting write_flags in the hook, I made it a
conditional "OR" just in case we'll extend write_flags later in common
paths and get it overlooked.

In short, I'll keep all zerocopy changes together in this single patch,
hopefully clearer.

=====
diff --git a/migration/multifd.c b/migration/multifd.c
index cd4467aff4..6aa44340de 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -50,15 +50,15 @@ typedef struct {
 /**
  * nocomp_send_setup: setup send side
  *
- * For no compression this function does nothing.
- *
- * Returns 0 for success or -1 for error
- *
  * @p: Params for the channel that we are using
  * @errp: pointer to an error
  */
 static int nocomp_send_setup(MultiFDSendParams *p, Error **errp)
 {
+    if (migrate_zero_copy_send()) {
+        p->write_flags |= QIO_CHANNEL_WRITE_FLAG_ZERO_COPY;
+    }
+
     return 0;
 }

-- 
Peter Xu