bhyve, Windows guest and trim support

[Andrea Venturoli]

Hello.

I'm running zvol-backed Windows guests in bhyve on FreeBSD 12.1.
Over time, I see the zvol effectively used space grow in size: a sign 
that Windows isn't trimming it.

I'm using virtio-blk: I guess this driver does not support trim?
I see reports that this support was added, but it's not clear to me 
whether it's in the released version or not (I've downloaded the stable 
binary ISO from RedHat).
Is trim expected to work in such a scenario?
If not, is it a bhyve problem or a guest-driver problem (or both)?

I tried virtio-scsi, which should support trim, but the guest doesn't 
even start in this case, getting stuck at the UEFI boot stage and ending 
up in UEFI prompt.
Again, is this expected?
No guest driver has been loaded yet, so is it a bhyve limitation?
Is some tweak needed?

I tried changing from virtio-blk to ahci-hd: I was then able to manually 
trim the disk from inside the guest and recover a lot of space in the zpool.
Alas ahci-hd is much slower, so I moved back to virtio-blk after that 
(at least for the time being).

  bye & Thanks
	av.