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Mon, 14 Oct 2019 14:09:31 +0000 (UTC) Subject: Re: [PATCH v3 07/16] qcow2: Write v3-compliant snapshot list on upgrade To: Eric Blake , qemu-block@nongnu.org References: <20191011152814.14791-1-mreitz@redhat.com> <20191011152814.14791-8-mreitz@redhat.com> <085c7aec-edd7-79bd-136f-5a3785a5d430@redhat.com> <582dc245-8a09-f88e-d67c-9435200c327d@redhat.com> 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: <61015d9f-7a85-c23f-aa3e-b6d85e7dfbcf@redhat.com> Date: Mon, 14 Oct 2019 16:09:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <582dc245-8a09-f88e-d67c-9435200c327d@redhat.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="RbCgoV33OIBSBLZARMq1jmQ0AFveH59BF" X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.62]); Mon, 14 Oct 2019 14:09:32 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 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: Kevin Wolf , qemu-devel@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) --RbCgoV33OIBSBLZARMq1jmQ0AFveH59BF Content-Type: multipart/mixed; boundary="BrwmBGeNUGeMwByUk4nWuRKHMlYd47ZXR" --BrwmBGeNUGeMwByUk4nWuRKHMlYd47ZXR Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 14.10.19 15:53, Eric Blake wrote: > On 10/14/19 3:45 AM, Max Reitz wrote: >=20 >>>> +=C2=A0=C2=A0=C2=A0 need_snapshot_update =3D false; >>>> +=C2=A0=C2=A0=C2=A0 for (i =3D 0; i < s->nb_snapshots; i++) { >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (s->snapshots[i].extr= a_data_size < >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 = sizeof_field(QCowSnapshotExtraData, vm_state_size_large) + >>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 = sizeof_field(QCowSnapshotExtraData, disk_size)) >>> >>> Shorter as: >>> if (s->snapshots[i].extra_data_size < sizeof(QCowSnapshotExtraData)) >>> >>> but that's stylistic, so R-b still stands. >> >> Yes, but if we ever add fields to QCowSnapshotExtraData, we shouldn=E2= =80=99t >> count them here.=C2=A0 Therefore, I think we need to count exactly the= fields >> that the standard says are mandatory in v3. >=20 > If we ever add more fields, I'd prefer that we did something like: >=20 > struct QCowSnapshotExtraV3Minimum { > =C2=A0=C2=A0=C2=A0 uint64_t vm_state_size_large; > =C2=A0=C2=A0=C2=A0 uint64_t disk_size; > }; > struct QCow3SnapshotExtraFull { > =C2=A0=C2=A0=C2=A0 struct QCowSnapshotExtraV3Minimum base; > =C2=A0=C2=A0=C2=A0 new fields...; > }; >=20 > and use sane naming to get at extra members based on the expected types= , > rather than trying to piecemeal portions of a type based on size. >=20 > Until we actually DO add more fields, why do we have to complicate the > current code? I don=E2=80=99t think it=E2=80=99s complicated, I find it very expressive= =2E There are two fields, we check whether they are present; why, that=E2=80=99s obviou= s, because those are the ones mandated by the standard. If we just checked against sizeof(QCowSnapshotExtraData), I=E2=80=99d (as= a na=C3=AFve reader) ask myself what that has to do with the standard. I=E2= =80=99d need to look into the structure definition and see that it currently contains exactly the fields that are mandated by the standard, and then I=E2=80=99d think =E2=80=9CBut what if we ever add fields to this structu= re?=E2=80=9D The more verbose version avoids this problem. And I=E2=80=99m not really inclined to take your proposal above right now= , because that would mean having to touch a lot of code. I prefer this more verbose code over that. Also, you explicitly agreed that the code in this patch is preferable to a plain sizeof(extra) in v2: https://lists.nongnu.org/archive/html/qemu-block/2019-08/msg00942.html Max --BrwmBGeNUGeMwByUk4nWuRKHMlYd47ZXR-- --RbCgoV33OIBSBLZARMq1jmQ0AFveH59BF Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl2kgZoACgkQ9AfbAGHV z0CW3wf/ZcjrU+7eBDutKstONW5u++u1okfmH7IYwagjUR3doFMfOEKDOs0Le/Ow i5X9X454vuBjuM5V4rQ0MvJHP/fBVNb2NU+UqB/pNJMx3iTXUXMMyAO4cZIocoWX wuwP0bp8777Mh6VCgbLzOrIj0M32tB83kFj47WnnmNjsZg4+lBaB9eC0VvZ5Nhyl Lw5L+I2BIySGPasuMamdd26rmVQPfJoby5AaM5O/4RYKtZE4+cEkVpPKCIHqeMQr svx+y4VAfYK6b+0b90b7nxaPRmZJ5/FeRrr9jUP42CATEyjVb5Tf1q8nzKXmMLW0 oOB9vAw5XF43VH1wFWIYj0LCd8uf6g== =wtc/ -----END PGP SIGNATURE----- --RbCgoV33OIBSBLZARMq1jmQ0AFveH59BF--