On 20.01.2017 17:25, Eric Farman wrote: > In the Linux kernel, I see two (three) places where the BLKSECTGET ioctl is > handled: > > (1) block/(compat_)ioctl.c -- (compat_)blkdev_ioctl > (2) drivers/scsi/sg.c -- sg_ioctl > > The former has been around forever[1], and returns a short value measured in > sectors. A sector is generally assumed to be 512 bytes. > > The latter has been around for slightly less than forever[2], and returns an > int that measures the value in bytes. A change to return the block count > was brought up a few years ago[3] and nacked. > > As a convenient example, if I use the blockdev tool to drive the ioctl to a > SCSI disk and its scsi-generic equivalent, I get different results: > > # lsscsi -g > [0:0:8:1077166114]disk IBM 2107900 .217 /dev/sda /dev/sg0 > # blockdev --getmaxsect /dev/sda > 2560 > # blockdev --getmaxsect /dev/sg0 > 20 > > Now, the value for /dev/sda looks "correct" to me. > > # cd /sys/devices/css0/0.0.0125/0.0.1f69/host0/rport-0\:0-8/ > # cd target0\:0\:8/0\:0\:8\:1077166114/ > # cat block/sda/queue/max_sectors_kb > 1280 > # cat block/sda/queue/hw_sector_size > 512 > > And the math checks out: > > max_sectors_kb * 1024 / hw_sector_size == getmaxsect > -OR- > 1280 * 1024 / 512 = 2560 > > For /dev/sg0, it appears the answer is coming from the sg_ioctl result > which is already multiplied by the block size, and then looking at only the > upper half (short) of the returned big-endian fullword: > > (1280 * 1024 / 512) * 512 = 1310720 = x00140000 => x0014 = 20 > > The reason for all this? Well, QEMU recently added a BLKSECTGET ioctl > call[4] which we see during guest boot. This code presumes the value is in > blocks/sectors, and converts it to bytes[5]. Not that this matters, because > the short/int discrepancy gives us "zero" on s390x. > > Also, that code doesn't execute for scsi-generic devices, so the conversion > to bytes is correct, but I'd like to extend this code to interrogate > scsi-generic devices as well. This is important because libvirt converts > a specified virtio-scsi device to its /dev/sgX address for the QEMU > commandline. > > Changes: > v2->v3: > - Move byte/sector conversions to patch 2 [Fam Zheng] > - Rename "max_sectors" when holding a byte value [Fam Zheng] > v1->v2: > - Patch 3, make hdev_get_max_transfer_length return bytes [Fam Zheng] > > [1] The initial kernel git commit > [2] kernel commit 44ec95425c1d9dce6e4638c29e4362cfb44814e7 > [3] https://lkml.org/lkml/2012/6/27/78 > [4] qemu commit 6f6071745bd0366221f5a0160ed7d18d0e38b9f7 > [5] qemu commit 5def6b80e1eca696c1fc6099e7f4d36729686402 > > Eric Farman (3): > hw/scsi: Fix debug message of cdb structure in scsi-generic > block: Fix target variable of BLKSECTGET ioctl > block: get max_transfer limit for char (scsi-generic) devices > > block/file-posix.c | 19 +++++++++++-------- > hw/scsi/scsi-generic.c | 5 +++-- > include/block/block.h | 1 + > 3 files changed, 15 insertions(+), 10 deletions(-)
Thank you, I've applied the series to my block tree (with the macro definitions in patch 2 switched as proposed): https://github.com/XanClic/qemu/commits/block Max
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