On Thu, Oct 08, 2026 at 12:06:47PM +0200, Leander Kohler wrote:
> The TSO and MTU fallback paths reset the device before renegotiating
> features. Reset clears all status bits, but these paths do not restore
> ACKNOWLEDGE and DRIVER before setting FEATURES_OK and DRIVER_OK.
> This violates the initialization sequence required by Virtio §3.1.1 [0].
> 
> We restore both bits after each fallback reset. This addresses one
> incompatibility with the Cloud Hypervisor VMM, which requires the
> complete initialization status before activating the device.
> 
> [0] https://docs.oasis-open.org/virtio/virtio/v1.4/cs01/virtio-v1.4-cs01.html

Thanks for your diff and pointing out this issue.  As far as I
understand the spec and our code, the following diff should also do the
trick and avoids redundant calls of ACK and DRIVER.  It also avoids
general VirtIO device behavior in a specific driver.

ok?

Thanks,
Jan

Index: dev/fdt/virtio_mmio.c
===================================================================
RCS file: /cvs/src/sys/dev/fdt/virtio_mmio.c,v
diff -u -p -r1.24 virtio_mmio.c
--- dev/fdt/virtio_mmio.c	22 Dec 2025 20:24:49 -0000	1.24
+++ dev/fdt/virtio_mmio.c	9 Oct 2026 22:01:53 -0000
@@ -305,9 +305,7 @@ virtio_mmio_attach(struct device *parent
 	vsc->sc_dmat = sc->sc_dmat;
 	sc->sc_config_offset = VIRTIO_MMIO_CONFIG;
 
-	virtio_device_reset(vsc);
-	virtio_mmio_set_status(vsc, VIRTIO_CONFIG_DEVICE_STATUS_ACK);
-	virtio_mmio_set_status(vsc, VIRTIO_CONFIG_DEVICE_STATUS_DRIVER);
+	virtio_reset(vsc);
 
 	vma.vma_va.va_devid = id;
 	vma.vma_va.va_nintr = 1;
Index: dev/pci/virtio_pci.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/virtio_pci.c,v
diff -u -p -r1.53 virtio_pci.c
--- dev/pci/virtio_pci.c	22 Dec 2025 20:24:49 -0000	1.53
+++ dev/pci/virtio_pci.c	9 Oct 2026 22:03:21 -0000
@@ -664,9 +664,7 @@ virtio_pci_attach(struct device *parent,
 		goto free;
 	}
 
-	virtio_device_reset(vsc);
-	virtio_set_status(vsc, VIRTIO_CONFIG_DEVICE_STATUS_ACK);
-	virtio_set_status(vsc, VIRTIO_CONFIG_DEVICE_STATUS_DRIVER);
+	virtio_reset(vsc);
 
 	printf("\n");
 	vpa.vpa_va.va_devid = id;
Index: dev/pv/virtio.c
===================================================================
RCS file: /cvs/src/sys/dev/pv/virtio.c,v
diff -u -p -r1.39 virtio.c
--- dev/pv/virtio.c	16 Sep 2025 12:18:10 -0000	1.39
+++ dev/pv/virtio.c	9 Oct 2026 21:45:34 -0000
@@ -152,6 +152,8 @@ virtio_reset(struct virtio_softc *sc)
 {
 	virtio_device_reset(sc);
 	sc->sc_active_features = 0;
+	virtio_set_status(sc, VIRTIO_CONFIG_DEVICE_STATUS_ACK);
+	virtio_set_status(sc, VIRTIO_CONFIG_DEVICE_STATUS_DRIVER);
 }
 
 int
@@ -180,8 +182,6 @@ virtio_reinit_start(struct virtio_softc 
 {
 	int i;
 
-	virtio_set_status(sc, VIRTIO_CONFIG_DEVICE_STATUS_ACK);
-	virtio_set_status(sc, VIRTIO_CONFIG_DEVICE_STATUS_DRIVER);
 	virtio_negotiate_features(sc, NULL);
 	sc->sc_ops->setup_intrs(sc);
 	for (i = 0; i < sc->sc_nvqs; i++) {