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Re: [RFC PATCH v2 2/8] qapi: net: introduce a way to bypass qemu_opts_pa
From: |
Markus Armbruster |
Subject: |
Re: [RFC PATCH v2 2/8] qapi: net: introduce a way to bypass qemu_opts_parse_noisily() |
Date: |
Wed, 15 Jun 2022 13:56:36 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) |
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