port-amd64/59728: nvmm(4): missing SMM/SMI support on x86

[campbell+netbsd%mumble.net@localhost]

>Number:         59728
>Category:       port-amd64
>Synopsis:       nvmm(4): missing SMM/SMI support on x86
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Oct 27 03:40:00 +0000 2025
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The SMMBSD Foundation
>Environment:
>Description:

	System Management Mode (SMM,
	https://en.wikipedia.org/wiki/System_Management_Mode) is a
	special mode of operation of x86 CPUs for use by firmware,
	which can be entered only through a System Management Interrupt
	(SMI), and which has a separate address space that is not
	accessible outside SMM.

	This is used even by virtualized firmware such as SeaBIOS, and
	it has come to be required (if accidentally) for certain
	functionality like VGA console in qemu (see
	https://gnats.NetBSD.org/59721, PR kern/59721: qemu VM's
	blackscreen over VNC wth -machine q35;
	https://gitlab.com/qemu-project/qemu/-/issues/2608).

	qemu's nvmm accelerator currently disables SMM because it
	requires hypervisor support, to notify qemu of VM exit due to
	SMI and to handle the separate address space.

>How-To-Repeat:

	qemu-system-x86_64 -machine q35,smm=on -accel nvmm

>Fix:

	Yes, please!