virtio: add loader tunables to sysctl

virtio_pci uses two loader tunables that should be more visible.
This patch adds these loader tunables to sysctl and describes them
in the virtio(4) man page.

Reviewed by:		imp (erlier version), tuexen
MFC after:		1 week
Differential Revision:	https://reviews.freebsd.org/D55533

share/man/man4/virtio.4

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 26, 2019
+.Dd March 2, 2026
 .Dt VIRTIO 4
 .Os
 .Sh NAME
@@ -54,7 +54,7 @@ The
 .Nm
 module provides a shared memory transport called a virtqueue.
 The
-.Xr virtio_pci
+.Sy virtio_pci
 device driver represents an emulated PCI device that the hypervisor makes
 available to the VM.
 This device provides the probing, configuration, and
@@ -92,6 +92,30 @@ An emulated SCSI HBA is provided by the
 .Xr virtio_scsi 4
 device driver.
 .El
+.Sh LOADER TUNABLES
+Tunables can be set at the
+.Xr loader 8
+prompt before booting the kernel or stored in
+.Xr loader.conf 5 .
+.Bl -tag -width "hw.virtio.pci.transitional"
+.It Va hw.virtio.pci.disable_msix
+If set to 1, disables MSI-X.
+The default value is 0.
+.It Va hw.virtio.pci.transitional
+For a transitional
+.Nm
+device, this tunable specifies whether to negotiate
+modern mode and use the modern
+.Nm
+driver
+.Pq 1
+or to negotiate legacy mode and
+use the legacy
+.Nm
+driver
+.Pq 1 .
+The default value is 0.
+.El
 .Sh SEE ALSO
 .Xr virtio_balloon 4 ,
 .Xr virtio_blk 4 ,

sys/dev/virtio/pci/virtio_pci.c

@@ -97,8 +97,13 @@ MODULE_VERSION(virtio_pci, 1);
 MODULE_DEPEND(virtio_pci, pci, 1, 1, 1);
 MODULE_DEPEND(virtio_pci, virtio, 1, 1, 1);
 
+SYSCTL_DECL(_hw_virtio);
+SYSCTL_NODE(_hw_virtio, OID_AUTO, pci, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
+    "VirtIO PCI driver parameters");
+
 int vtpci_disable_msix = 0;
-TUNABLE_INT("hw.virtio.pci.disable_msix", &vtpci_disable_msix);
+SYSCTL_INT(_hw_virtio_pci, OID_AUTO, disable_msix, CTLFLAG_RDTUN,
+    &vtpci_disable_msix, 0, "If set to 1, disables MSI-X.");
 
 static uint8_t
 vtpci_read_isr(struct vtpci_common *cn)

sys/dev/virtio/pci/virtio_pci_modern.c

@@ -33,6 +33,7 @@
 #include <sys/bus.h>
 #include <sys/lock.h>
 #include <sys/kernel.h>
+#include <sys/sysctl.h>
 #include <sys/module.h>
 
 #include <machine/bus.h>
@@ -188,8 +189,12 @@ static void	vtpci_modern_write_device_8(struct vtpci_modern_softc *,
 		    bus_size_t, uint64_t);
 
 /* Tunables. */
+SYSCTL_DECL(_hw_virtio_pci);
+
 static int vtpci_modern_transitional = 0;
-TUNABLE_INT("hw.virtio.pci.transitional", &vtpci_modern_transitional);
+SYSCTL_INT(_hw_virtio_pci, OID_AUTO, transitional, CTLFLAG_RDTUN,
+    &vtpci_modern_transitional, 0,
+    "If 0, a transitional VirtIO device is used in legacy mode; otherwise, in modern mode.");
 
 static device_method_t vtpci_modern_methods[] = {
 	/* Device interface. */

sys/dev/virtio/virtio.c

@@ -30,6 +30,7 @@
 #include <sys/systm.h>
 #include <sys/kernel.h>
 #include <sys/malloc.h>
+#include <sys/sysctl.h>
 #include <sys/module.h>
 #include <sys/sbuf.h>
 
@@ -90,6 +91,9 @@ static struct virtio_feature_desc virtio_common_feature_desc[] = {
 	{ 0, NULL }
 };
 
+SYSCTL_NODE(_hw, OID_AUTO, virtio, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
+    "VirtIO driver parameters");
+
 const char *
 virtio_device_name(uint16_t devid)
 {