[pve-devel] [PATCH qemu-server] cpu config: nested-virt: always recommend a CPU model similar to the host

[Fiona Ebner <f.ebner@proxmox.com>]

Not only for Windows, nested virtualization with just x86-64-v2-AES
and the nested-virt flag does not work on Linux either. While
virtualization support is detected, actually starting a nested VM with
such a configuration fails. For example, a Proxmox VE/PBS file-restore
VM on my test system fails with the following (adding the apic and
x2apic flags on top wouldn't make a difference):

[2025-11-19T10:37:44+01:00] PBS file restore VM log
ERROR: Unable to locate IOAPIC for GSI 2
ERROR: Unable to locate IOAPIC for GSI 5
ERROR: Unable to locate IOAPIC for GSI 9
ERROR: Unable to locate IOAPIC for GSI 10
ERROR: Unable to locate IOAPIC for GSI 11
ERROR: Unable to locate IOAPIC for GSI 1
ERROR: Unable to locate IOAPIC for GSI 2
ERROR: Unable to locate IOAPIC for GSI 3
ERROR: Unable to locate IOAPIC for GSI 4
ERROR: Unable to locate IOAPIC for GSI 5
ERROR: Unable to locate IOAPIC for GSI 6
ERROR: Unable to locate IOAPIC for GSI 7
ERROR: Unable to locate IOAPIC for GSI 8
ERROR: Unable to locate IOAPIC for GSI 9
ERROR: Unable to locate IOAPIC for GSI 10
ERROR: Unable to locate IOAPIC for GSI 11
ERROR: Unable to locate IOAPIC for GSI 12
ERROR: Unable to locate IOAPIC for GSI 13
ERROR: Unable to locate IOAPIC for GSI 14
ERROR: Unable to locate IOAPIC for GSI 15
..MP-BIOS bug: 8254 timer not connected to IO-APIC
Kernel panic - not syncing: IO-APIC + timer doesn't work!  Boot with apic=debug and send a report.  Then try booting with the 'noapic' option.
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G        W          6.14.6-pbs-restore #1
Tainted: [W]=WARN
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x4a/0x70
 panic+0xf5/0x281
 ? _printk+0x55/0x60
 setup_IO_APIC+0x6ee/0x6f0
 ? __ioapic_read_entry+0x23/0x40
 ? clear_IO_APIC_pin+0x11e/0x1e0
 apic_intr_mode_init+0xad/0xc0
 x86_late_time_init+0x12/0x30
 start_kernel+0x538/0x5b0
 x86_64_start_reservations+0x25/0x30
 x86_64_start_kernel+0x73/0x80
 common_startup_64+0x129/0x138
 </TASK>

Reported-by: Lukas Wagner <l.wagner@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 src/PVE/QemuServer/CPUConfig.pm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PVE/QemuServer/CPUConfig.pm b/src/PVE/QemuServer/CPUConfig.pm
index c0d4c9a4..b63743be 100644
--- a/src/PVE/QemuServer/CPUConfig.pm
+++ b/src/PVE/QemuServer/CPUConfig.pm
@@ -170,7 +170,7 @@ my $supported_cpu_flags = [
         name => 'nested-virt',
         description => "Controls nested virtualization, namely 'svm' for AMD CPUs and 'vmx' for"
             . " Intel CPUs. Live migration still only works if it's the same flag on both sides."
-            . " For Windows, use a CPU model similar to the host, with the same vendor!",
+            . " Use a CPU model similar to the host, with the same vendor, not x86-64-vX!",
     },
     {
         name => 'md-clear',
-- 
2.47.3



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