Don't add any default devices and any default USB controller models for
the machine type and add few basic tests.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
---
 src/qemu/qemu_command.c                       |  1 +
 src/qemu/qemu_domain.c                        | 16 ++++++++-
 .../microvm.x86_64-latest.args                | 34 +++++++++++++++++++
 .../qemuxmlconfdata/microvm.x86_64-latest.xml | 29 ++++++++++++++++
 tests/qemuxmlconfdata/microvm.xml             | 20 +++++++++++
 ...troller-implicit-microvm.x86_64-latest.err |  1 +
 ...troller-implicit-microvm.x86_64-latest.xml | 25 ++++++++++++++
 .../usb-controller-implicit-microvm.xml       | 17 ++++++++++
 ...rovm-minimal.x86_64-latest.abi-update.args | 31 +++++++++++++++++
 ...crovm-minimal.x86_64-latest.abi-update.xml | 23 +++++++++++++
 .../x86_64-microvm-minimal.x86_64-latest.args | 31 +++++++++++++++++
 .../x86_64-microvm-minimal.x86_64-latest.xml  | 23 +++++++++++++
 .../x86_64-microvm-minimal.xml                | 14 ++++++++
 tests/qemuxmlconftest.c                       |  4 +++
 14 files changed, 268 insertions(+), 1 deletion(-)
 create mode 100644 tests/qemuxmlconfdata/microvm.x86_64-latest.args
 create mode 100644 tests/qemuxmlconfdata/microvm.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/microvm.xml
 create mode 100644 tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.err
 create mode 100644 tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/usb-controller-implicit-microvm.xml
 create mode 100644 tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.args
 create mode 100644 tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.xml
 create mode 100644 tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.args
 create mode 100644 tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/x86_64-microvm-minimal.xml

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 2719574fb5..d0fa31a506 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -2923,6 +2923,7 @@ static bool
 qemuBuildDomainForbidLegacyUSBController(const virDomainDef *def)
 {
     if (qemuDomainIsQ35(def) ||
+        qemuDomainIsMicrovm(def) ||
         qemuDomainIsARMVirt(def) ||
         qemuDomainIsRISCVVirt(def))
         return true;
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 25cb650c07..aa58af8c22 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -3911,7 +3911,8 @@ virXMLNamespace virQEMUDriverDomainXMLNamespace = {
 static int
 qemuDomainDefAddImplicitInputDevice(virDomainDef *def)
 {
-    if (ARCH_IS_X86(def->os.arch)) {
+    if (ARCH_IS_X86(def->os.arch) &&
+        !qemuDomainIsMicrovm(def)) {
         if (virDomainDefMaybeAddInput(def,
                                       VIR_DOMAIN_INPUT_TYPE_MOUSE,
                                       VIR_DOMAIN_INPUT_BUS_PS2) < 0)
@@ -4157,6 +4158,14 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
     switch (def->os.arch) {
     case VIR_ARCH_I686:
     case VIR_ARCH_X86_64:
+        /* don't add anything for microvm */
+        if (qemuDomainIsMicrovm(def)) {
+            /* explicitly add 'none' USB controller */
+            usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE;
+            addDefaultUSB = true;
+            break;
+        }
+
         addDefaultMemballoon = true;

         if (STREQ(def->os.machine, "isapc")) {
@@ -5665,6 +5674,11 @@ qemuDomainControllerDefPostParse(virDomainControllerDef *cont,
                     cont->model = VIR_DOMAIN_CONTROLLER_MODEL_USB_QEMU_XHCI;
                 else if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_NEC_USB_XHCI))
                     cont->model = VIR_DOMAIN_CONTROLLER_MODEL_USB_NEC_XHCI;
+            } else if (ARCH_IS_X86(def->os.arch)) {
+                if (qemuDomainIsMicrovm(def)) {
+                    /* do not select any automatic model for 'microvm' machines */
+                    cont->model = -1;
+                }
             }
         }
         /* forbid usb model 'qusb1' and 'qusb2' in this kind of hyperviosr */
diff --git a/tests/qemuxmlconfdata/microvm.x86_64-latest.args b/tests/qemuxmlconfdata/microvm.x86_64-latest.args
new file mode 100644
index 0000000000..a2ee9ce3a4
--- /dev/null
+++ b/tests/qemuxmlconfdata/microvm.x86_64-latest.args
@@ -0,0 +1,34 @@
+LC_ALL=C \
+PATH=/bin \
+HOME=/var/lib/libvirt/qemu/domain--1-microvm \
+USER=test \
+LOGNAME=test \
+XDG_DATA_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.local/share \
+XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.cache \
+XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.config \
+/usr/bin/qemu-system-x86_64 \
+-name guest=microvm,debug-threads=on \
+-S \
+-object '{"qom-type":"secret","id":"masterKey0","format":"raw","file":"/var/lib/libvirt/qemu/domain--1-microvm/master-key.aes"}' \
+-machine microvm,usb=off,dump-guest-core=off,memory-backend=microvm.ram,acpi=off \
+-accel kvm \
+-cpu qemu64 \
+-m size=1024000k \
+-object '{"qom-type":"memory-backend-ram","id":"microvm.ram","size":1048576000}' \
+-overcommit mem-lock=off \
+-smp 1,sockets=1,cores=1,threads=1 \
+-uuid e739ac15-61b5-48c2-acc8-e7fb2b79774f \
+-display none \
+-no-user-config \
+-nodefaults \
+-chardev socket,id=charmonitor,fd=1729,server=on,wait=off \
+-mon chardev=charmonitor,id=monitor,mode=control \
+-rtc base=utc \
+-no-shutdown \
+-boot strict=on \
+-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/microvm.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \
+-blockdev '{"node-name":"libvirt-1-format","read-only":false,"driver":"qcow2","file":"libvirt-1-storage"}' \
+-device '{"driver":"virtio-blk-device","drive":"libvirt-1-format","id":"virtio-disk0","bootindex":1}' \
+-audiodev '{"id":"audio1","driver":"none"}' \
+-sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
+-msg timestamp=on
diff --git a/tests/qemuxmlconfdata/microvm.x86_64-latest.xml b/tests/qemuxmlconfdata/microvm.x86_64-latest.xml
new file mode 100644
index 0000000000..24a4eccf83
--- /dev/null
+++ b/tests/qemuxmlconfdata/microvm.x86_64-latest.xml
@@ -0,0 +1,29 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <cpu mode='custom' match='exact' check='none'>
+    <model fallback='forbid'>qemu64</model>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <disk type='file' device='disk'>
+      <driver name='qemu' type='qcow2'/>
+      <source file='/var/lib/libvirt/images/microvm.qcow2'/>
+      <target dev='vda' bus='virtio'/>
+      <address type='virtio-mmio'/>
+    </disk>
+    <controller type='usb' index='0' model='none'/>
+    <audio id='1' type='none'/>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/microvm.xml b/tests/qemuxmlconfdata/microvm.xml
new file mode 100644
index 0000000000..4aa576b0bc
--- /dev/null
+++ b/tests/qemuxmlconfdata/microvm.xml
@@ -0,0 +1,20 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <disk type='file' device='disk'>
+      <driver name='qemu' type='qcow2'/>
+      <source file='/var/lib/libvirt/images/microvm.qcow2'/>
+      <target dev='vda' bus='virtio'/>
+      <address type='virtio-mmio'/>
+    </disk>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.err b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.err
new file mode 100644
index 0000000000..7a71aa107d
--- /dev/null
+++ b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.err
@@ -0,0 +1 @@
+unsupported configuration: no model provided for USB controller
diff --git a/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.xml b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.xml
new file mode 100644
index 0000000000..50b3c1e908
--- /dev/null
+++ b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.x86_64-latest.xml
@@ -0,0 +1,25 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <cpu mode='custom' match='exact' check='none'>
+    <model fallback='forbid'>qemu64</model>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0'>
+      <address type='virtio-mmio'/>
+    </controller>
+    <audio id='1' type='none'/>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/usb-controller-implicit-microvm.xml b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.xml
new file mode 100644
index 0000000000..f4c28e0f79
--- /dev/null
+++ b/tests/qemuxmlconfdata/usb-controller-implicit-microvm.xml
@@ -0,0 +1,17 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0'>
+      <address type='virtio-mmio'/>
+    </controller>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.args b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.args
new file mode 100644
index 0000000000..12e8302a85
--- /dev/null
+++ b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.args
@@ -0,0 +1,31 @@
+LC_ALL=C \
+PATH=/bin \
+HOME=/var/lib/libvirt/qemu/domain--1-microvm \
+USER=test \
+LOGNAME=test \
+XDG_DATA_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.local/share \
+XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.cache \
+XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.config \
+/usr/bin/qemu-system-x86_64 \
+-name guest=microvm,debug-threads=on \
+-S \
+-object '{"qom-type":"secret","id":"masterKey0","format":"raw","file":"/var/lib/libvirt/qemu/domain--1-microvm/master-key.aes"}' \
+-machine microvm,usb=off,dump-guest-core=off,memory-backend=microvm.ram,acpi=off \
+-accel kvm \
+-cpu qemu64 \
+-m size=1024000k \
+-object '{"qom-type":"memory-backend-ram","id":"microvm.ram","size":1048576000}' \
+-overcommit mem-lock=off \
+-smp 1,sockets=1,cores=1,threads=1 \
+-uuid e739ac15-61b5-48c2-acc8-e7fb2b79774f \
+-display none \
+-no-user-config \
+-nodefaults \
+-chardev socket,id=charmonitor,fd=1729,server=on,wait=off \
+-mon chardev=charmonitor,id=monitor,mode=control \
+-rtc base=utc \
+-no-shutdown \
+-boot strict=on \
+-audiodev '{"id":"audio1","driver":"none"}' \
+-sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
+-msg timestamp=on
diff --git a/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.xml b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.xml
new file mode 100644
index 0000000000..287eb3f58e
--- /dev/null
+++ b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.abi-update.xml
@@ -0,0 +1,23 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <cpu mode='custom' match='exact' check='none'>
+    <model fallback='forbid'>qemu64</model>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0' model='none'/>
+    <audio id='1' type='none'/>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.args b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.args
new file mode 100644
index 0000000000..12e8302a85
--- /dev/null
+++ b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.args
@@ -0,0 +1,31 @@
+LC_ALL=C \
+PATH=/bin \
+HOME=/var/lib/libvirt/qemu/domain--1-microvm \
+USER=test \
+LOGNAME=test \
+XDG_DATA_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.local/share \
+XDG_CACHE_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.cache \
+XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-microvm/.config \
+/usr/bin/qemu-system-x86_64 \
+-name guest=microvm,debug-threads=on \
+-S \
+-object '{"qom-type":"secret","id":"masterKey0","format":"raw","file":"/var/lib/libvirt/qemu/domain--1-microvm/master-key.aes"}' \
+-machine microvm,usb=off,dump-guest-core=off,memory-backend=microvm.ram,acpi=off \
+-accel kvm \
+-cpu qemu64 \
+-m size=1024000k \
+-object '{"qom-type":"memory-backend-ram","id":"microvm.ram","size":1048576000}' \
+-overcommit mem-lock=off \
+-smp 1,sockets=1,cores=1,threads=1 \
+-uuid e739ac15-61b5-48c2-acc8-e7fb2b79774f \
+-display none \
+-no-user-config \
+-nodefaults \
+-chardev socket,id=charmonitor,fd=1729,server=on,wait=off \
+-mon chardev=charmonitor,id=monitor,mode=control \
+-rtc base=utc \
+-no-shutdown \
+-boot strict=on \
+-audiodev '{"id":"audio1","driver":"none"}' \
+-sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
+-msg timestamp=on
diff --git a/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.xml b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.xml
new file mode 100644
index 0000000000..287eb3f58e
--- /dev/null
+++ b/tests/qemuxmlconfdata/x86_64-microvm-minimal.x86_64-latest.xml
@@ -0,0 +1,23 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <cpu mode='custom' match='exact' check='none'>
+    <model fallback='forbid'>qemu64</model>
+  </cpu>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <controller type='usb' index='0' model='none'/>
+    <audio id='1' type='none'/>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconfdata/x86_64-microvm-minimal.xml b/tests/qemuxmlconfdata/x86_64-microvm-minimal.xml
new file mode 100644
index 0000000000..f0d8e54494
--- /dev/null
+++ b/tests/qemuxmlconfdata/x86_64-microvm-minimal.xml
@@ -0,0 +1,14 @@
+<domain type='kvm'>
+  <name>microvm</name>
+  <uuid>e739ac15-61b5-48c2-acc8-e7fb2b79774f</uuid>
+  <memory unit='KiB'>1024000</memory>
+  <currentMemory unit='KiB'>1024000</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='microvm'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+  </devices>
+</domain>
diff --git a/tests/qemuxmlconftest.c b/tests/qemuxmlconftest.c
index 03453e8ec8..33f185dbc9 100644
--- a/tests/qemuxmlconftest.c
+++ b/tests/qemuxmlconftest.c
@@ -1238,6 +1238,8 @@ mymain(void)
     DO_TEST_CAPS_LATEST_ABI_UPDATE("x86_64-pc-minimal");
     DO_TEST_CAPS_LATEST("x86_64-q35-minimal");
     DO_TEST_CAPS_LATEST_ABI_UPDATE("x86_64-q35-minimal");
+    DO_TEST_CAPS_LATEST("x86_64-microvm-minimal");
+    DO_TEST_CAPS_LATEST_ABI_UPDATE("x86_64-microvm-minimal");
     DO_TEST_CAPS_ARCH_LATEST("aarch64-virt-minimal", "aarch64");
     DO_TEST_CAPS_ARCH_LATEST_ABI_UPDATE("aarch64-virt-minimal", "aarch64");
     DO_TEST_CAPS_ARCH_LATEST("riscv64-virt-minimal", "riscv64");
@@ -1839,6 +1841,7 @@ mymain(void)
     DO_TEST_CAPS_LATEST("usb-controller-implicit-isapc");
     DO_TEST_CAPS_LATEST("usb-controller-implicit-i440fx");
     DO_TEST_CAPS_LATEST("usb-controller-implicit-q35");
+    DO_TEST_CAPS_LATEST_FAILURE("usb-controller-implicit-microvm");
     DO_TEST_CAPS_LATEST_PARSE_ERROR("usb-controller-default-isapc");
     DO_TEST_CAPS_LATEST("usb-controller-default-i440fx");
     DO_TEST_CAPS_LATEST("usb-controller-default-q35");
@@ -2365,6 +2368,7 @@ mymain(void)
     DO_TEST_CAPS_LATEST("pci-bridge-many-disks");
     DO_TEST_CAPS_LATEST("pcie-root");
     DO_TEST_CAPS_LATEST("q35");
+    DO_TEST_CAPS_LATEST("microvm");
     DO_TEST_CAPS_LATEST_PARSE_ERROR("q35-dmi-bad-address1");
     DO_TEST_CAPS_LATEST_PARSE_ERROR("q35-dmi-bad-address2");
     DO_TEST_CAPS_LATEST("pc-i440fx-acpi-root-hotplug-disable");
-- 
2.43.0
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On Tue, Feb 27, 2024 at 05:36:08PM +0100, Peter Krempa wrote:
> @@ -4157,6 +4158,14 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
>      switch (def->os.arch) {
>      case VIR_ARCH_I686:
>      case VIR_ARCH_X86_64:
> +        /* don't add anything for microvm */
> +        if (qemuDomainIsMicrovm(def)) {
> +            /* explicitly add 'none' USB controller */
> +            usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE;
> +            addDefaultUSB = true;
> +            break;
> +        }

I'm not terribly keen on seeing support for microvm officially
introduced in libvirt.

AFAIK nobody ever asked for it, and given that the intended use case
for the machine type is specifically to have incredibly minimal, fast
booting VMs, to the extent where trimmed out kernels and even custom
firmware implementations are employed to shave every last possible
microsecond off the boot time, I really don't expect that anyone
would ever consider using libvirt to manage them.

In other words, I think we should just keep pretending they don't
exist and save a bit of code/complexity.

-- 
Andrea Bolognani / Red Hat / Virtualization
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On Wed, Feb 28, 2024 at 06:35:46AM -0800, Andrea Bolognani wrote:
> On Tue, Feb 27, 2024 at 05:36:08PM +0100, Peter Krempa wrote:
> > @@ -4157,6 +4158,14 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
> >      switch (def->os.arch) {
> >      case VIR_ARCH_I686:
> >      case VIR_ARCH_X86_64:
> > +        /* don't add anything for microvm */
> > +        if (qemuDomainIsMicrovm(def)) {
> > +            /* explicitly add 'none' USB controller */
> > +            usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE;
> > +            addDefaultUSB = true;
> > +            break;
> > +        }
> 
> I'm not terribly keen on seeing support for microvm officially
> introduced in libvirt.
> 
> AFAIK nobody ever asked for it, and given that the intended use case
> for the machine type is specifically to have incredibly minimal, fast
> booting VMs, to the extent where trimmed out kernels and even custom
> firmware implementations are employed to shave every last possible
> microsecond off the boot time, I really don't expect that anyone
> would ever consider using libvirt to manage them.

What you describe is one use case for microvm, but more generally
I'd say microvm provides a "legacy free" machine type. Q35 is still
a legacy platform, just a little less legacy than i440fx.

By eliminating legacy platform features, microvm reduces the guest
machine attack surface. As such, I think it is conceptially relevant
to want to use microvm for general purpose guest OS, while not caring
about its supposed performance differences.

With regards,
Daniel
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On Wed, Feb 28, 2024 at 02:56:02PM +0000, Daniel P. Berrangé wrote:
> On Wed, Feb 28, 2024 at 06:35:46AM -0800, Andrea Bolognani wrote:
> > On Tue, Feb 27, 2024 at 05:36:08PM +0100, Peter Krempa wrote:
> > > @@ -4157,6 +4158,14 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
> > >      switch (def->os.arch) {
> > >      case VIR_ARCH_I686:
> > >      case VIR_ARCH_X86_64:
> > > +        /* don't add anything for microvm */
> > > +        if (qemuDomainIsMicrovm(def)) {
> > > +            /* explicitly add 'none' USB controller */
> > > +            usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE;
> > > +            addDefaultUSB = true;
> > > +            break;
> > > +        }
> >
> > I'm not terribly keen on seeing support for microvm officially
> > introduced in libvirt.
> >
> > AFAIK nobody ever asked for it, and given that the intended use case
> > for the machine type is specifically to have incredibly minimal, fast
> > booting VMs, to the extent where trimmed out kernels and even custom
> > firmware implementations are employed to shave every last possible
> > microsecond off the boot time, I really don't expect that anyone
> > would ever consider using libvirt to manage them.
>
> What you describe is one use case for microvm, but more generally
> I'd say microvm provides a "legacy free" machine type. Q35 is still
> a legacy platform, just a little less legacy than i440fx.
>
> By eliminating legacy platform features, microvm reduces the guest
> machine attack surface. As such, I think it is conceptially relevant
> to want to use microvm for general purpose guest OS, while not caring
> about its supposed performance differences.

By default, microvm is too opinionated to be considered usable as a
general-purpose machine type. For example, it uses virtio-mmio
instead of PCI, so no hotplug out of the box. It can be massaged into
something more suitable through the use of additional options, but
really at that point it would IMO make a lot more sense to introduce
a 'virt' machine type similar to the one that aarch64 and riscv64
already have, leaving microvm to its original use case.

-- 
Andrea Bolognani / Red Hat / Virtualization
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On Wed, Feb 28, 2024 at 06:35:46 -0800, Andrea Bolognani wrote:
> On Tue, Feb 27, 2024 at 05:36:08PM +0100, Peter Krempa wrote:
> > @@ -4157,6 +4158,14 @@ qemuDomainDefAddDefaultDevices(virQEMUDriver *driver,
> >      switch (def->os.arch) {
> >      case VIR_ARCH_I686:
> >      case VIR_ARCH_X86_64:
> > +        /* don't add anything for microvm */
> > +        if (qemuDomainIsMicrovm(def)) {
> > +            /* explicitly add 'none' USB controller */
> > +            usbModel = VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE;
> > +            addDefaultUSB = true;
> > +            break;
> > +        }
> 
> I'm not terribly keen on seeing support for microvm officially
> introduced in libvirt.
> 
> AFAIK nobody ever asked for it, and given that the intended use case
> for the machine type is specifically to have incredibly minimal, fast
> booting VMs, to the extent where trimmed out kernels and even custom
> firmware implementations are employed to shave every last possible
> microsecond off the boot time, I really don't expect that anyone
> would ever consider using libvirt to manage them.
> 
> In other words, I think we should just keep pretending they don't
> exist and save a bit of code/complexity.

I sure can do that. My motivation for doing this was to cover explicitly
every case (machine type) where qemu does honour '-usb'. 'microvm' is
one of them (if ACPI is enabled) and it adds a MMIO mapped USB
controller.

Regardless of whether this patch gets applied the USB controller will
be removed from there as we don't really support the platform-internal
USB controllers for ARM boards either.
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