On Wed, 2019-01-16 at 12:44 +0000, Daniel P. Berrangé wrote:
On Wed, Jan 16, 2019 at 01:29:13PM +0100, Andrea Bolognani wrote:
On Wed, 2019-01-16 at 11:39 +0000, Daniel P. Berrangé wrote:
In the future they may add properties to, or change the defaults on, the -transitional or -non-transitional devices only, associated with new machine type versions. If libvirt forever uses the old devices, then we loose ability to take advantage of that.
Regardless of what libvirt ends up doing, from the QEMU user point of view I think it would be very surprising if eg. virtio-blk-pci plugged into a PCIe slot behaved differently from virtio-blk-pci-non-transitional plugged into the very same slot, or if virtio-net-pci,disable-legacy=false,disable-modern=false behaved differently from virtio-net-pci-transitional regardless of the slot it's plugged into, so moving away from that consistency should be a non-goal IMHO.
[...]
In this message Eduardo said virtio-blk-pci,disable-legacy and virtio-blk-pci-non-transitional are only compatible with the pc-4.0 machine types and earlier. There's no compat guarantee of compat for future machine types:
https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg03762.html
If we didn't use the new QEMU device models right now, we could end up trapped forever. The safe futureproof approach is to always use the new devices models if available, and use disable-legacy for old QEMU versions only, which we know will have old machine types for which the compat guarantee is available.
Well, let's see if Eduardo is willing to reconsider the policy on compatibility between virtio-*-pci-{,non-}transitional and plain virtio-*-pci going forward based on the principle-of-least-surprise rationale outlined above :) -- Andrea Bolognani / Red Hat / Virtualization