Re: [PATCH Libvirt 1/3] qemu_capabilities: Introduce virtio-blk DISCARD and WRITE_ZEROES capabilities

On 7/17/23 21:43, Jonathon Jongsma wrote: Just for completeness, since Hyman is new contributor: this fact Jonathon mentions is necessary but not sufficient. If a capability depends on compile time/runtime configuration then we still want to have capability. For instance, we have QEMU_CAPS_SPICE which tells whether QEMU was built with SPICE support. Now, SPICE was introduced to QEMU somewhere in 0.14 (according to [1]) which is way older than 4.2.0 (~8 years difference), and yet we still have a capability for it, because QEMU can be built without SPICE support. But this is not the case for '.discard' and '.write-zeroes' attributes you are introducing as they are declared unconditionally in QEMU code: hw/block/virtio-blk.c=1706=static Property virtio_blk_properties[] = { -- hw/block/virtio-blk.c-1725- DEFINE_PROP_BIT64("discard", VirtIOBlock, host_features, hw/block/virtio-blk.c-1726- VIRTIO_BLK_F_DISCARD, true), hw/block/virtio-blk.c-1727- DEFINE_PROP_BOOL("report-discard-granularity", VirtIOBlock, hw/block/virtio-blk.c-1728- conf.report_discard_granularity, true), hw/block/virtio-blk.c:1729: DEFINE_PROP_BIT64("write-zeroes", VirtIOBlock, host_features, hw/block/virtio-blk.c-1730- VIRTIO_BLK_F_WRITE_ZEROES, true), and both were introduced in QEMU commit v4.0.0-rc0~80^2~7. IOW, this capability would be always set and thus is redundant. BTW: to check the minimal version required by libvirt you can use plain grep: libvirt.git $ grep QEMU_MIN_ src/qemu/qemu_capabilities.c #define QEMU_MIN_MAJOR 4 #define QEMU_MIN_MINOR 2 #define QEMU_MIN_MICRO 0 1: https://wiki.qemu.org/ChangeLog/0.14 Michal