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Mon, 17 Aug 2020 13:53:59 +0000 (UTC) Subject: Re: [RFC PATCH 04/22] block/export: Add BlockExport infrastructure and block-export-add To: Kevin Wolf References: <20200813162935.210070-1-kwolf@redhat.com> <20200813162935.210070-5-kwolf@redhat.com> <7ed669db-7a75-fb25-4ce6-52369ea01b4b@redhat.com> <20200817124544.GI11402@linux.fritz.box> <0a8939b5-4441-e76e-44c5-b27e69eba3b8@redhat.com> <20200817132938.GM11402@linux.fritz.box> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <4fef913a-986b-7f64-c8e5-0bb45c5fc05f@redhat.com> Date: Mon, 17 Aug 2020 15:53:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200817132938.GM11402@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="qFhrdmAYYnAJ1CQmxvP3bNcXBMXnNvukg" Received-SPF: pass client-ip=205.139.110.61; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/17 05:03:47 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --qFhrdmAYYnAJ1CQmxvP3bNcXBMXnNvukg Content-Type: multipart/mixed; boundary="nimiRosnp6IN9KcGhjE29ELg71tneumRk" --nimiRosnp6IN9KcGhjE29ELg71tneumRk Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 17.08.20 15:29, Kevin Wolf wrote: > Am 17.08.2020 um 15:19 hat Max Reitz geschrieben: >> On 17.08.20 14:45, Kevin Wolf wrote: >>> Am 17.08.2020 um 12:03 hat Max Reitz geschrieben: >>>> On 13.08.20 18:29, Kevin Wolf wrote: >>>>> We want to have a common set of commands for all types of block expor= ts. >>>>> Currently, this is only NBD, but we're going to add more types. >>>>> >>>>> This patch adds the basic BlockExport and BlockExportDriver structs a= nd >>>>> a QMP command block-export-add that creates a new export based on the >>>>> given BlockExportOptions. >>>>> >>>>> qmp_nbd_server_add() becomes a wrapper around qmp_block_export_add(). >>>>> >>>>> Signed-off-by: Kevin Wolf >>>>> --- >>>>> qapi/block-export.json | 9 ++++++ >>>>> include/block/export.h | 32 +++++++++++++++++++++ >>>>> include/block/nbd.h | 3 +- >>>>> block/export/export.c | 57 ++++++++++++++++++++++++++++++++++++= ++ >>>>> blockdev-nbd.c | 19 ++++++++----- >>>>> nbd/server.c | 15 +++++++++- >>>>> Makefile.objs | 6 ++-- >>>>> block/Makefile.objs | 2 ++ >>>>> block/export/Makefile.objs | 1 + >>>>> 9 files changed, 132 insertions(+), 12 deletions(-) >>>>> create mode 100644 include/block/export.h >>>>> create mode 100644 block/export/export.c >>>>> create mode 100644 block/export/Makefile.objs >>>> >>>> Nothing of too great importance below. But it=E2=80=99s an RFC, so co= mments I >>>> will give. >>>> >>>>> diff --git a/block/export/export.c b/block/export/export.c >>>>> new file mode 100644 >>>>> index 0000000000..3d0dacb3f2 >>>>> --- /dev/null >>>>> +++ b/block/export/export.c >>>>> @@ -0,0 +1,57 @@ >>>>> +/* >>>>> + * Common block export infrastructure >>>>> + * >>>>> + * Copyright (c) 2012, 2020 Red Hat, Inc. >>>>> + * >>>>> + * Authors: >>>>> + * Paolo Bonzini >>>>> + * Kevin Wolf >>>>> + * >>>>> + * This work is licensed under the terms of the GNU GPL, version 2 o= r >>>>> + * later. See the COPYING file in the top-level directory. >>>>> + */ >>>>> + >>>>> +#include "qemu/osdep.h" >>>>> + >>>>> +#include "block/export.h" >>>>> +#include "block/nbd.h" >>>>> +#include "qapi/error.h" >>>>> +#include "qapi/qapi-commands-block-export.h" >>>>> + >>>>> +static const BlockExportDriver* blk_exp_drivers[] =3D { >>>> ^^ >>>> Sternenplatzierung *hust* >>>> >>>>> + &blk_exp_nbd, >>>>> +}; >>>> >>>> Not sure whether I like this better than the block driver way of >>>> registering block drivers with a constructor. It requires writing les= s >>>> code, at the expense of making the variable global. So I think there= =E2=80=99s >>>> no good reason to prefer the block driver approach. >>> >>> I guess I can see one reason why we may want to switch to the >>> registration style eventually: If we we want to make export drivers >>> optional modules which may or may not be present. >> >> Good point. >> >>>> Maybe my hesitance comes from the variable being declared (as extern) = in >>>> a header file (block/export.h). I think I would prefer it if we put >>>> that external reference only here in this file. Would that work, or d= o >>>> you have other plans that require blk_exp_nbd to be visible outside of >>>> nbd/server.c and this file here? >>> >>> Hm, do we have precedence for "public, but not really" variables? >>> Normally I expect public symbols to be declared in a header file. >> >> Hm, yes. >> >> tl;dr: I was wrong about a local external reference being nicer. But I >> believe there is a difference between externally-facing header files >> (e.g. block.h) and internal header files (e.g. block_int.h). I don=E2= =80=99t >> know which of those block/export.h is supposed to be. >> >> (And of course it doesn=E2=80=99t even matter at all, really.) >> >> >> non-tl;dr: >> >> We have a similar case for bdrv_{file,raw,qcow2}, but those are at least >> in a *_int.h. I can=E2=80=99t say I like that style. >> >> OK, let me try to figure out what my problem with this is. >> >> I think if a module (in this case the NBD export code) exports >> something, it should be available in the respective header (i.e., some >> NBD header), not in some other header. A module=E2=80=99s header should= present >> what it exports to the rest of the code. The export code probably >> doesn=E2=80=99t want to export the NBD driver object, it wants to import= it, >> actually. So if it should be in a header file, it should be in an NBD >> header. >> >> Now none of our block drivers has a header file for exporting symbols to >> the rest of the block code, which is why their symbols have been put >> into block_int.h. I think that=E2=80=99s cutting corners, but can be de= fended >> by saying that block_int.h is not for exporting anything, but just >> collects stuff internal to the block layer, so it kind of fits there. >> >> (Still, technically, I believe bdrv_{file,raw,qcow2} should be exported >> by each respective block driver in a driver-specific header file. If >> that isn=E2=80=99t the case, it doesn=E2=80=99t really matter to me whet= her it=E2=80=99s put >> into a dedicated header file to collect internal stuff (block_int.h) or >> just imported locally (with an external declaration) where it=E2=80=99s = used. >> Probably the dedicated header file is cleaner after all, right.) >> >> Maybe block/export.h is the same in that it=E2=80=99s just supposed to c= ollect >> symbols used internally by the export code, then it isn=E2=80=99t wrong = to put >> it there. But if it=E2=80=99s a header file that may be used by non-exp= ort code >> to use export functionality, then it would be wrong. >> >> But whatever. >> >> Now I have sorted out my feelings, and they don=E2=80=99t give any resul= t at >> all, but it was kind of therapeutic for me. >=20 > Actually, there could be a conclusion: The declaration shouldn't be in > include/block/export.h, but in include/block/nbd.h. We already include > both headers in block/export/export.c because of qmp_nbd_*(). Sounds good. > Of course, you already requests that I leave the other NBD-related stuff > in blockdev-nbd.c rather than moving it there, so the use of blk_exp_nbd > would be the only reason that remains for export.c to include nbd.h. Aww. That=E2=80=99s too bad. > But it might still be better than having it in export.h. I=E2=80=99ll take the easy way out and leave it to you. Max --nimiRosnp6IN9KcGhjE29ELg71tneumRk-- --qFhrdmAYYnAJ1CQmxvP3bNcXBMXnNvukg Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl86i/YACgkQ9AfbAGHV z0DoMwf+PtAZLPlnC/+F/EKbmCsceEwTjMB6tY7IwgklIJRnNmCKDlGkvDEo0ypZ 6HBYHMv7TW+2IAdTRNfAeMEZgyxec7pUtXunaDwZDvPNoHcGry/ElEJXlBvfMnhw Ip3VFcsIXArR81743VTPdJ6gCDDNfeYbpTPiyRx7bHh0Dl0apSjSbAhJgdcodKjY aq+PXcA1277SZTtcngYnPGPuMp+wc7i97nR22LIlffnnbg5rHNGvrBRhDRc4oYyo CnOM/USljQWzV/yCrbQK3TzRXka2Am5EgySsvPCD8v+yW5NUmQRVlFwpmZSS6sU1 6dtxltEGxndt7oyzM0pkspPWs7rw4Q== =/4ev -----END PGP SIGNATURE----- --qFhrdmAYYnAJ1CQmxvP3bNcXBMXnNvukg--