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Wed, 30 Oct 2019 16:23:34 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-147.ams2.redhat.com [10.36.117.147]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 23F715C1B0; Wed, 30 Oct 2019 16:23:31 +0000 (UTC) Subject: Re: [RFC PATCH v2 05/26] qcow2: Document the Extended L2 Entries feature To: Alberto Garcia , qemu-devel@nongnu.org References: <57cddd5db9c79406fba70a9bf46c436f6fc9ccd3.1572125022.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: <0c738172-732f-eee0-3396-5a9576a4addf@redhat.com> Date: Wed, 30 Oct 2019 17:23:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <57cddd5db9c79406fba70a9bf46c436f6fc9ccd3.1572125022.git.berto@igalia.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="xbQxQrHqHXV31FpQHYVHkRejA605Kxa4i" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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) --xbQxQrHqHXV31FpQHYVHkRejA605Kxa4i Content-Type: multipart/mixed; boundary="HY8rDvDpsSvc9nb1X63Hm611XOEVksutR" --HY8rDvDpsSvc9nb1X63Hm611XOEVksutR Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 26.10.19 23:25, 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(-) Sounds good, just a bit of bit nit-picking from my side. > 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 [...] > @@ -524,6 +535,57 @@ file (except if bit 0 in the Standard Cluster Descri= ptor is set). If there is > no backing file or the backing file is smaller than the image, they shal= l read > zeros for all parts that are not covered by the backing file. > =20 > +=3D=3D Extended L2 Entries =3D=3D > + > +An image uses Extended L2 Entries if bit 3 is set on the incompatible_fe= atures > +field of the header. > + > +In these images standard data clusters are divided into 32 subclusters o= f the > +same size. They are contiguous and start from the beginning of the clust= er. > +Subclusters can be allocated independently and the L2 entry contains inf= ormation > +indicating the status of each one of them. Compressed data clusters don'= t have > +subclusters so they are treated like in images without this feature. > + > +The size of an extended L2 entry is 128 bits so the number of entries pe= r table > +is calculated using this formula: > + > + l2_entries =3D (cluster_size / (2 * sizeof(uint64_t))) > + > +The first 64 bits have the same format as the standard L2 table entry de= scribed > +in the previous section, with the exception of bit 0 of the standard clu= ster > +descriptor. > + > +The last 64 bits contain a subcluster allocation bitmap with this format= : > + > +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 seem to remember that someone proposed this bit ordering to you, but I wonder why. So far everything in qcow2 starts from the least significant bit, for example refcounts (=E2=80=9CIf refcount_bits implies a sub-byte width, note that bit 0 means the least significant bit in this context=E2=80=9D), feature bits, and sub-byte structure descriptions in gen= eral (which you reference directly with =E2=80=9Cbit x=E2=80=9D). Soo... What=E2=80=99s the reason for doing it the other way around here? > + 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) > + > +Subcluster Allocation Bitmap (for compressed clusters): Going from nit-picking to bike shedding: I=E2=80=99d drop the parentheses. 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