xen: improve man (4) page

Update the xen(4) man page to reflect the current support status.

Reported by: kevans
Reviewed by: bcr kevans imp
Differential revision: https://reviews.freebsd.org/D43373

1 files changed, 33 insertions, 41 deletions

diff --git a/share/man/man4/xen.4 b/share/man/man4/xen.4
index 6660ab3c268b..15312866dae6 100644
--- a/share/man/man4/xen.4
+++ b/share/man/man4/xen.4
@@ -26,19 +26,19 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 30, 2015
+.Dd January 8, 2024
 .Dt XEN 4
 .Os
 .Sh NAME
 .Nm xen
-.Nd Xen Hypervisor Guest (DomU) Support
+.Nd Xen Hypervisor Support
 .Sh SYNOPSIS
-To compile hardware-assisted virtualization (HVM) Xen guest support with
-para-virtualized drivers into an amd64 or i386 kernel,
-place the following lines in your kernel configuration file:
-.Bd -ragged -offset indent
-.Cd "options XENHVM"
-.Cd "device xenpci"
+FreeBSD supports running both as a Xen guest and host on amd64 hardware.
+Guest support is limited to HVM and PVH modes, while host support is limited to
+PVH mode only.
+.Pp
+Xen support is built by default in the i386 and amd64 GENERIC kernels; note
+however that host mode is only available on amd64.
 .Ed
 .Sh DESCRIPTION
 The Xen Hypervisor allows multiple virtual machines to be run on a single
@@ -52,20 +52,18 @@ to access resources such as virtual network interfaces and disk devices.
 .Pp
 With later instruction set extensions from AMD and Intel to support fully
 virtualizable instructions, unmodified virtual memory systems can also be
-supported; this is referred to as hardware-assisted virtualization (HVM).
+supported; this is referred to as hardware-assisted virtualization (HVM and PVH).
 HVM configurations may either rely on transparently emulated hardware
 peripherals, or para-virtualized drivers, which are aware of virtualization,
 and hence able to optimize certain behaviors to improve performance or
 semantics.
+PVH configurations rely on para-virtualized drivers exclusively for IO.
 .Pp
 .Fx
-supports hardware-assisted virtualization (HVM) on both i386 and amd64
-kernels.
-.Pp
 Para-virtualized device drivers are required in order to support certain
 functionality, such as processing management requests, returning idle
 physical memory pages to the hypervisor, etc.
-.Ss Xen DomU device drivers
+.Ss Xen device drivers
 These para-virtualized drivers are supported:
 .Bl -hang -offset indent -width blkfront
 .It Nm balloon
@@ -87,6 +85,10 @@ suspend, crash, and halt requests.
 Expose Xen events via the
 .Pa /dev/xen/evtchn
 special device.
+.It Nm gntdev
+Allow access to the grant table interface via the
+.Pa /dev/xen/gntdev
+special device.
 .It Nm netback
 Export local network interfaces to other Xen domains where they can be
 imported via
@@ -94,35 +96,30 @@ imported via
 .It Nm netfront
 Import network interfaces from other Xen domains as local network interfaces,
 which may be used for IPv4, IPv6, etc.
-.It Nm pcifront
-Allow physical PCI devices to be passed through into a PV domain.
+.It Nm privcmd
+Allow issuing hypercalls via the
+.Pa /dev/xen/privcmd
+special device.
+.It Nm timer
+Implementation of a one-shot high resolution per-CPU timer using the hypercall
+interface.
+.It Nm acpi cpu
+When running as a host forwards power management related information from ACPI
+to the hypervisor for better performance management.
 .It Nm xenpci
 Represents the Xen PCI device, an emulated PCI device that is exposed to
 HVM domains.
 This device allows detection of the Xen hypervisor, and provides interrupt
 and shared memory services required to interact with the hypervisor.
+.It Nm xenstore
+Information storage space shared between domains.
 .El
-.Ss Performance considerations
-In general, PV drivers will perform better than emulated hardware, and are
-the recommended configuration for HVM installations.
-.Pp
-Using a hypervisor introduces a second layer of scheduling that may limit the
-effectiveness of certain
-.Fx
-scheduling optimisations.
-Among these is adaptive locking, which is no longer able to determine whether
-a thread holding a lock is in execution.
-It is recommended that adaptive locking be disabled when using Xen:
-.Bd -unfilled -offset indent
-.Cd "options NO_ADAPTIVE_MUTEXES"
-.Cd "options NO_ADAPTIVE_RWLOCKS"
-.Cd "options NO_ADAPTIVE_SX"
-.Ed
 .Sh HISTORY
 Support for
 .Nm
 first appeared in
 .Fx 8.1 .
+Support for host mode was added in 11.0 .
 .Sh AUTHORS
 .An -nosplit
 .Fx
@@ -133,15 +130,10 @@ and
 Further refinements were made by
 .An Justin Gibbs Aq Mt gibbs@FreeBSD.org ,
 .An Adrian Chadd Aq Mt adrian@FreeBSD.org ,
+.An Colin Percival Aq Mt cperciva@FreeBSD.org ,
 and
-.An Colin Percival Aq Mt cperciva@FreeBSD.org .
+.An Roger Pau Monné Aq Mt royger@FreeBSD.org .
 This manual page was written by
-.An Robert Watson Aq Mt rwatson@FreeBSD.org .
-.Sh BUGS
-.Fx
-is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0).
-.Pp
-As of this release, Xen PV DomU support is not heavily tested; instability
-has been reported during VM migration of PV kernels.
-.Pp
-Certain PV driver features, such as the balloon driver, are under-exercised.
+.An Robert Watson Aq Mt rwatson@FreeBSD.org ,
+and
+.An Roger Pau Monné Aq Mt royger@FreeBSD.org .