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Thu, 20 Feb 2020 15:54:22 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.36.118.38]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7163589F08; Thu, 20 Feb 2020 15:54:16 +0000 (UTC) Subject: Re: [RFC PATCH v3 05/27] qcow2: Document the Extended L2 Entries feature To: Alberto Garcia , qemu-devel@nongnu.org References: <0b884ddcd0ac3a3c0b8cdd9d09c74566ac107c9a.1577014346.git.berto@igalia.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: <7ff19f65-5148-a40a-9b7a-6a330cf7272e@redhat.com> Date: Thu, 20 Feb 2020 16:54:09 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <0b884ddcd0ac3a3c0b8cdd9d09c74566ac107c9a.1577014346.git.berto@igalia.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="YP0xxjwwPFhk4laozEnLD82eDYv78gwHQ" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 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 , Anton Nefedov , qemu-block@nongnu.org, Vladimir Sementsov-Ogievskiy , "Denis V . Lunev" 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) --YP0xxjwwPFhk4laozEnLD82eDYv78gwHQ Content-Type: multipart/mixed; boundary="brAosk83YhrqLffY1Nwcm1fbwuOfW2iN2" --brAosk83YhrqLffY1Nwcm1fbwuOfW2iN2 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 22.12.19 12:36, Alberto Garcia wrote: > Subcluster allocation in qcow2 is implemented by extending the > existing L2 table entries and adding additional information to > indicate the allocation status of each subcluster. >=20 > This patch documents the changes to the qcow2 format and how they > affect the calculation of the L2 cache size. >=20 > Signed-off-by: Alberto Garcia > --- > docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++-- > docs/qcow2-cache.txt | 19 +++++++++++- > 2 files changed, 83 insertions(+), 4 deletions(-) >=20 > diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt > index af5711e533..d34261f955 100644 > --- a/docs/interop/qcow2.txt > +++ b/docs/interop/qcow2.txt > @@ -39,6 +39,9 @@ The first cluster of a qcow2 image contains the file he= ader: > as the maximum cluster size and won't be able to ope= n images > with larger cluster sizes. > =20 > + Note: if the image has Extended L2 Entries then clus= ter_bits > + must be at least 14 (i.e. 16384 byte clusters). > + > 24 - 31: size > Virtual disk size in bytes. > =20 > @@ -109,7 +112,12 @@ in the description of a field. > An External Data File Name header extens= ion may > be present if this bit is set. > =20 > - Bits 3-63: Reserved (set to 0) > + Bit 3: Extended L2 Entries. If this bit is set= then I suppose bit 4 now. (Compression is bit 3.) [...] > +Subcluster Allocation Bitmap (for standard clusters): > + > + Bit 0 - 31: Allocation status (one bit per subcluster) > + > + 1: the subcluster is allocated. In this case the > + host cluster offset field must contain a valid > + offset. > + 0: the subcluster is not allocated. In this case > + read requests shall go to the backing file or > + return zeros if there is no backing file data. > + > + Bits are assigned starting from the most significant= one. > + (i.e. bit x is used for subcluster 31 - x) I still prefer it the other way round, both personally (e.g. it=E2=80=99s t= he C ordering), and because other places in qcow2 use LSb for bit ordering (the refcount order). I don=E2=80=99t see ease of debugging as a particularly good reason; but th= en again, I didn=E2=80=99t have to debug this feature yet (as opposed to you). But since I=E2=80=99m used to counting bits from the right (because this is= how it=E2=80=99s done basically everywhere), I can=E2=80=99t imagine I would fi= nd it more difficult than counting them from the left. Max > + 32 - 63 Subcluster reads as zeros (one bit per subcluster) > + > + 1: the subcluster reads as zeros. In this case the > + allocation status bit must be unset. The host > + cluster offset field may or may not be set. > + 0: no effect. > + > + Bits are assigned starting from the most significant= one. > + (i.e. bit x is used for subcluster 63 - x) --brAosk83YhrqLffY1Nwcm1fbwuOfW2iN2-- --YP0xxjwwPFhk4laozEnLD82eDYv78gwHQ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl5Oq6EACgkQ9AfbAGHV z0Dwdgf/ds9m+41IbzNA+uSQAlfUybGynxkLhYz4c1I20UaJqHNnD5FgccdrzmES ccwcTGt1NnW8cblKpvoAwk9djB1uBSL2C5a1DFVN1WFaF5Yc6Tcxyr5Ktj67HPOn K+ifTebdKva3zyhwUFWp5HI+F8nrlyrAYVJr439VgYZobVD8nvgk8effQ7lw1cR9 c2K0vdff81q0ua81jUF6kLW7Sc85TbCizYI3Bd5zymiLl6l2V1vOE+2vdQf3g3Uv pYkbX9q37WOvqF3hxHzV+/iFJQAETaZawhUqjzdv+VK1E2pn61L73pebVj9DbiLi kSncwGGNcwUKqCj2hPcZ4SxFbkbVdw== =Btqr -----END PGP SIGNATURE----- --YP0xxjwwPFhk4laozEnLD82eDYv78gwHQ--