Re: [Qemu-devel] [PATCH 4/4] migration: simplify writev vs. non-writev logic

[Paolo Bonzini]

Il 09/04/2013 13:43, Juan Quintela ha scritto:
>> > @@ -687,12 +685,10 @@ void qemu_put_byte(QEMUFile *f, int v)
>> >      f->bytes_xfer++;
>> >      if (f->ops->writev_buffer) {
>> >          add_to_iovec(f, f->buf + f->buf_index, 1);
>> > -        f->buf_index++;
>> > -    } else {
>> > -        f->buf_index++;
>> > -        if (f->buf_index == IO_BUF_SIZE) {
>> > -            qemu_fflush(f);
>> > -        }
>> > +    }
>> > +    f->buf_index++;
>> > +    if (f->buf_index == IO_BUF_SIZE) {
>> > +        qemu_fflush(f);
>> >      }
>> >  }
> If you follow my advice of moving the call to add_to_iovec() you get
> this one simplified and only one place to do this.

Moving what call?  The apparent complication is because the old logic
was a bit more involute than necessary.  If you look at the code after
the patches, not the patches themselves, you'll see for yourself.

The logic now is:

   add byte
   if using iovs
       add byte to iov list
   if buffer full
       flush

add_to_iovec has no business checking the buffer.  Why should
qemu_put_buffer_async() check the buffer?

The duplication between qemu_put_byte and qemu_put_buffer is a different
topic.  I think it's acceptable in the name of performance, but perhaps
you can just call qemu_put_buffer(f, &c, 1).

Paolo