Re: [PATCH v3 6/8] tap: cpr support

[Markus Armbruster <armbru@redhat.com>]

Jason Wang <jasowang@redhat.com> writes:

> Hi Markus,
>
> On Thu, Dec 4, 2025 at 4:09 PM Markus Armbruster <armbru@redhat.com> wrote:
>>
>> Ben Chaney <bchaney@akamai.com> writes:
>>
>> > From: Steve Sistare <steven.sistare@oracle.com>
>> >
>> > Provide the cpr=on option to preserve TAP and vhost descriptors during
>> > cpr-transfer, so the management layer does not need to create a new
>> > device for the target.
>> >
>> > Save all tap fd's in canonical order, leveraging the index argument of
>> > cpr_save_fd.  For the i'th queue, the tap device fd is saved at index 2*i,
>> > and the vhostfd (if any) at index 2*i+1.
>> >
>> > tap and vhost fd's are passed by name to the monitor when a NIC is hot
>> > plugged, but the name is not known to qemu after cpr.  Allow the manager
>> > to pass -1 for the fd "name" in the new qemu args to indicate that QEMU
>> > should search for a saved value.  Example:
>> >
>> >   -netdev tap,id=hostnet2,fds=-1:-1,vhostfds=-1:-1,cpr=on
>>
>> Hmm.  See below.
>>
>> >
>> > Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> > Signed-off-by: Ben Chaney <bchaney@akamai.com>
>>
>> [...]
>>
>> > diff --git a/qapi/net.json b/qapi/net.json
>> > index 118bd34965..264213b5d9 100644
>> > --- a/qapi/net.json
>> > +++ b/qapi/net.json
>> > @@ -355,6 +355,8 @@
>>    ##
>>    # @NetdevTapOptions:
>>    #
>>    # Used to configure a host TAP network interface backend.
>>    #
>>    # @ifname: interface name
>>    #
>>    # @fd: file descriptor of an already opened tap
>>    #
>>    # @fds: multiple file descriptors of already opened multiqueue capable
>>    #     tap
>>
>> Not this patch's fault: the interface is misguided, and its
>> documentation inadequate.
>>
>> @fds is a string of file descriptor names or numbers separated by ':'.
>> Not documented.  I found out by reading the code.
>>
>> This violates QAPI design principle "no string parsing".  It should be
>> an array of strings.
>>
>
> I agree with your concern. Just a note that this "fds" was introduced
> before QAPI if I am not wrong.

It's from 2013 (commit 264986e2c8f).  QAPI was still young then
(netdev_add had been QAPIfied less than a year ago), we had much to
learn, and interface review barely happened.  All understandable, and no
reason to throw shade on anyone involved :)