Re: [RFC PATCH v2 2/8] qapi: net: introduce a way to bypass qemu_opts_parse_noisily()

[Markus Armbruster <armbru@redhat.com>]

Laurent Vivier <lvivier@redhat.com> writes:

> On 13/05/2022 13:21, Markus Armbruster wrote:
>> Laurent Vivier <lvivier@redhat.com> writes:
>> 
>>> As qemu_opts_parse_noisily() flattens the QAPI structures ("type" field
>>> of Netdev structure can collides with "type" field of SocketAddress),
>> 
>> To remember how this works, I have to write a more verbose version of
>> the above.  Why not post it then, so here goes.
>> 
>> qemu_init() passes the argument of -netdev, -nic, and -net to
>> net_client_parse().
>> 
>> net_client_parse() parses with qemu_opts_parse_noisily(), passing
>> QemuOptsList qemu_netdev_opts for -netdev, qemu_nic_opts for -nic, and
>> qemu_net_opts for -net.  Their desc[] are all empty, which means any
>> keys are accepted.  The result of the parse (a QemuOpts) is stored in
>> the QemuOptsList.
>> 
>> Note that QemuOpts is flat by design.  In some places, we layer non-flat
>> on top using dotted keys convention, but not here.
>> 
>> net_init_clients() iterates over the stored QemuOpts, and passes them to
>> net_init_netdev(), net_param_nic(), or net_init_client(), respectively.
>> 
>> These functions pass the QemuOpts to net_client_init().  They also do
>> other things with the QemuOpts, which we can ignore here.
>> 
>> net_client_init() uses the opts visitor to convert the (flat) QemOpts to
>> a (non-flat) QAPI object Netdev.  Netdev is also the argument of QMP
>> command netdev_add.
>> 
>> The opts visitor was an early attempt to support QAPI in
>> (QemuOpts-based) CLI.  It restricts QAPI types to a certain shape; see
>> commit eb7ee2cbeb "qapi: introduce OptsVisitor".
>> 
>> A more modern way to support QAPI is qobject_input_visitor_new_str().
>> It uses keyval_parse() instead of QemuOpts for KEY=VALUE,... syntax, and
>> it also supports JSON syntax.  The former isn't quite as expressive as
>> JSON, but it's a lot closer than QemuOpts + opts visitor.
>> 
>>> we introduce a way to bypass qemu_opts_parse_noisily() and use directly
>>> visit_type_Netdev() to parse the backend parameters.
>> 
>> This commit paves the way to use of the modern way instead.
>
> I'm going to copy your analysis to the commit message of the patch.

Go right ahead :)

>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>> ---
>>>   net/net.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>   1 file changed, 54 insertions(+)
>>>
>>> diff --git a/net/net.c b/net/net.c
>>> index 58c05c200622..2aab7167316c 100644
>>> --- a/net/net.c
>>> +++ b/net/net.c
>>> @@ -54,6 +54,7 @@
>>>   #include "net/colo-compare.h"
>>>   #include "net/filter.h"
>>>   #include "qapi/string-output-visitor.h"
>>> +#include "qapi/qobject-input-visitor.h"
>>>   
>>>   /* Net bridge is currently not supported for W32. */
>>>   #if !defined(_WIN32)
>>> @@ -63,6 +64,17 @@
>>>   static VMChangeStateEntry *net_change_state_entry;
>>>   static QTAILQ_HEAD(, NetClientState) net_clients;
>>>   
>>> +typedef struct NetdevQueueEntry {
>>> +    bool is_netdev;
>>> +    Netdev *nd;
>>> +    Location loc;
>>> +    QSIMPLEQ_ENTRY(NetdevQueueEntry) entry;
>>> +} NetdevQueueEntry;
>>> +
>>> +typedef QSIMPLEQ_HEAD(, NetdevQueueEntry) NetdevQueue;
>>> +
>>> +static NetdevQueue nd_queue = QSIMPLEQ_HEAD_INITIALIZER(nd_queue);
>>> +
>>>   /***********************************************************/
>>>   /* network device redirectors */
>>>   
>>> @@ -1559,6 +1571,19 @@ int net_init_clients(Error **errp)
>>>   
>>>       QTAILQ_INIT(&net_clients);
>>>   
>>> +    while (!QSIMPLEQ_EMPTY(&nd_queue)) {
>>> +        NetdevQueueEntry *nd = QSIMPLEQ_FIRST(&nd_queue);
>>> +
>>> +        QSIMPLEQ_REMOVE_HEAD(&nd_queue, entry);
>>> +        loc_push_restore(&nd->loc);
>>> +        if (net_client_init1(nd->nd, nd->is_netdev, errp) < 0) {
>> 
>> I think you need to loc_pop() here.
>> 
>>> +            return -1;
>>> +        }
>> 
>> Since the only caller passes &error_fatal, I'd be tempted to ditch the
>> @errp argument, and simply do
>> 
>>             net_client_init1(nd->nd, nd->is_netdev, &error_fatal);
>> 
>> It's what we do for -blockdev, -device, and -object.
>
> I've added a patch to remove the @errp from the net_init_clients() arguments.
>
>> 
>>> +        loc_pop(&nd->loc);
>>> +        qapi_free_Netdev(nd->nd);
>>> +        g_free(nd);
>>> +    }
>>> +
>>>       if (qemu_opts_foreach(qemu_find_opts("netdev"),
>>>                             net_init_netdev, NULL, errp)) {
>>>           return -1;
>>> @@ -1575,8 +1600,37 @@ int net_init_clients(Error **errp)
>>>       return 0;
>>>   }
>>>   
>>> +/*
>>> + * netdev_is_modern() returns true when the backend needs to bypass
>>> + * qemu_opts_parse_noisily()
>>> + */
>>> +static bool netdev_is_modern(const char *optarg)
>>> +{
>>> +    return false;
>>> +}
>>> +
>>>   int net_client_parse(QemuOptsList *opts_list, const char *optarg)
>>>   {
>>> +    if (netdev_is_modern(optarg)) {
>>> +            /*
>>> +             * We need to bypass qemu_opts_parse_noisily() to accept
>>> +             * new style object like addr.type=inet in SocketAddress
>>> +             */
>> 
>> I'm not sure this will makes sense to future readers.
>> 
>> What about "Use modern, more expressive syntax"?
>
> Done.
>
>> 
>>> +            Visitor *v;
>>> +            NetdevQueueEntry *nd;
>>> +
>>> +            v = qobject_input_visitor_new_str(optarg, "type",
>>> +                                              &error_fatal);
>>> +            nd = g_new(NetdevQueueEntry, 1);
>>> +            visit_type_Netdev(v, NULL, &nd->nd, &error_fatal);
>>> +            visit_free(v);
>>> +            loc_save(&nd->loc);
>>> +            nd->is_netdev = strcmp(opts_list->name, "netdev") == 0;
>>> +
>>> +            QSIMPLEQ_INSERT_TAIL(&nd_queue, nd, entry);
>>> +            return 0;
>>> +    }
>> 
>> Matches what we do for -blockdev, except we additionally have
>> nd->is_netdev.  We need it for calling net_client_init1().
>> 
>> If netdev_is_modern(optarg), then the only use of parameter @opts_list
>> is opts_list->name in the initialization of nd->is_netdev.
>> 
>> There's a bit of code smell, I'm afraid.
>
> I don't see what is the problem.

The function's signature

    int net_client_parse(QemuOptsList *opts_list, const char *optarg)

suggests we're parsing @optarg into @opts_list.

We do only if !netdev_is_modern(optarg).  In other words, the function's
actual behavior doesn't match reasonable expectations based on
signature.  A function comment would mitigate.

If nd->is_netdev wasn't needed, we could code all this just like
-blockdev.  I think that would be simpler.

>> I believe @is_netdev needs to be true for -netdev, -nic, and netdev_add;
>> false for -net.
>> 
>> Will we ever use the modern syntax with -net?
>
> Yes, I think we should support the same syntax with -netdev and -net.
>
> My first iteration was to pass is_netdev=true to net_client_init1() and Stefano reported a 
> problem with "-net" with things like that:
>
>      -net dgram,id=socket0,local.type=inet,local.host=localhost,local.port=1234,\
>                            remote.type=inet,remote.host=localhost,remote.port=1235
>      -net nic,model=virtio,macaddr=9a:2b:2c:2d:2e:2f
>
>> 
>> Any chance we can deprecate -net?
>
> Who can decide of that?

Thomas, I think you've done some work to replace use cases of -net.  Do
you know what's left?

>>> +
>>>       if (!qemu_opts_parse_noisily(opts_list, optarg, true)) {
>>>           return -1;
>>>       }
>> 
>
> Thanks,
> Laurent