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Mon, 17 Aug 2020 13:19:38 +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> 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: <0a8939b5-4441-e76e-44c5-b27e69eba3b8@redhat.com> Date: Mon, 17 Aug 2020 15:19:37 +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: <20200817124544.GI11402@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Z6K91vAIoMVOGEt4kAmPQG8OzdM07lvea" Received-SPF: pass client-ip=207.211.31.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/17 00:24:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 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_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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) --Z6K91vAIoMVOGEt4kAmPQG8OzdM07lvea Content-Type: multipart/mixed; boundary="y3EXDafofW1qrYri1hM5hfN2YZOZSUECk" --y3EXDafofW1qrYri1hM5hfN2YZOZSUECk Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable 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 exports= . >>> Currently, this is only NBD, but we're going to add more types. >>> >>> This patch adds the basic BlockExport and BlockExportDriver structs and >>> 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 comm= ents 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 or >>> + * 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 less >> 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. >=20 > 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 do >> you have other plans that require blk_exp_nbd to be visible outside of >> nbd/server.c and this file here? >=20 > 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 pr= esent 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 defen= ded 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 whether= 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 use= d. 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 coll= ect 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-export= 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 result a= t all, but it was kind of therapeutic for me. >>> +static const BlockExportDriver *blk_exp_find_driver(BlockExportType ty= pe) >>> +{ >>> + int i; >>> + >>> + for (i =3D 0; i < ARRAY_SIZE(blk_exp_drivers); i++) { >>> + if (blk_exp_drivers[i]->type =3D=3D type) { >>> + return blk_exp_drivers[i]; >>> + } >>> + } >> >> How bad would it be to define blk_exp_drivers as >> blk_exp_drivers[BLOCK_EXPORT_TYPE__MAX] and use the BlockExportType as >> the driver index so we don=E2=80=99t have to loop here? >> >> Not that it matters performance-wise. Just something I wondered. >=20 > Might be nicer indeed. It would be incompatible with a registration > model, though, so if we're not sure yet what we want to have in the long > term, maybe the more neutral way is to leave it as it is. Yes, true. >>> + return NULL; >> >> Why not e.g. g_assert_not_reached()? >> >> (If the BlockExportType were used as the index, I=E2=80=99d assert that >> type < ARRAY_SIZE(blk_exp_drivers) && blk_exp_drivers[type] !=3D NULL. = I >> don=E2=80=99t think there=E2=80=99s a reason for graceful handling.) >=20 > Same thing actually. This works as long as all drivers are always > present. >=20 > Now I understand that the current state is somewhat inconsistent in that > it uses a simple array of things that are always present, but has > functions that work as if it were dynamic. I don't mind this > inconsistency very much, but if you do, I guess I could implement a > registration type thing right away. Sounds all reasonable. Thus, I=E2=80=99d leave it like you did it and care about a registration model if/when we need it. Reviewed-by: Max Reitz --y3EXDafofW1qrYri1hM5hfN2YZOZSUECk-- --Z6K91vAIoMVOGEt4kAmPQG8OzdM07lvea Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl86g+kACgkQ9AfbAGHV z0ADtggAq2ZHMn+geEii70ibD045YQvla1tEoZCsCRYhn1yLNvQIN1Olt0sxHopS SvYVAGnUKGI3fvTJcmtCShKPxXOrpFIuBVbk9expY5nWxkvbwRqy7v3EtmdSEJaU BWDMCz3619CMuzaOT151H4TsopUJBo1oiSuCq6JvNZdK8HbIO4LUp7t2Hactjpdd jaroVHXCffAH5skCuONUqxZP1LJ0SAi0btv3QsejgqCk5ocNpkv2NVfCNlrsRwot JDP6diXU3qrgko2sIdA64kXQ1Pcne+RPhV76vFHkql+fm1mc3TdIGMl0U4fRufcB QZ0/1VkUh73oAX9xYcjtEccobtSaLQ== =gN2/ -----END PGP SIGNATURE----- --Z6K91vAIoMVOGEt4kAmPQG8OzdM07lvea--