(cc'ing linux-scsi for the cover-letter; patches only to QEMU lists.) 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. So, do I have to code around the different field sizes (int vs short) as well as scaling (bytes vs blocks)? Obviously doable, but looking at the resulting commits, I find myself feeling a little ill. [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 | 16 +++++++++++++--- hw/scsi/scsi-generic.c | 5 +++-- 2 files changed, 16 insertions(+), 5 deletions(-) -- 2.8.4 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
