Reporting of rotation rate for disks

While looking at libvirt tagged questions on serverfault I saw someone ask how to indicate that a virtual disk is an SSD rather than rotating rust.

https://serverfault.com/questions/876467/how-to-add-virtual-storage-as-ssd-in-kvm

IIUC, IDE / SCSI don't really have an explicit concept of a disk being SSD vs HDD, but they do have a means of reporting the disk rotation rate and both specify that SSDs should report RPM==1.

Linux uses this to determine whether to set the 'rotational' property to 0 for the I/O queue, and also whether to allow the disk to contribute random entropy (only HDDs contribute).

This felt like something QEMU ought to allow the mgmt application to set based on the storage it is using to back the virtual block devices. So this series adds a 'rotation_rate' property to the SCSI and IDE disks, taking an RPM value per their respective specifications.

There is no mechanism to report this information to virtio-blk. We could perhaps argue that people should use virtio-scsi instead, because fixing virtio-blk would require enhancement to both QEMU and Linux virtio-blk drivers (and other guest OS drivers too)

I'm unclear if there is anything I should have done wrt the device migration vmstate when adding this extra field, or if it is safe to just expect the mgmt app to set the property correctly on src+dst ?

Daniel P. Berrange (2):
scsi-disk: support reporting of rotation rate
ide: support reporting of rotation rate

 hw/ide/core.c             |  1 +
 hw/ide/qdev.c             |  1 +
 hw/scsi/scsi-disk.c       | 20 ++++++++++++++++++++
 include/hw/ide/internal.h |  8 ++++++++
 4 files changed, 30 insertions(+)

2.13.5