qemu-x86: kernel panic when host is loaded

[Corentin Labbe]

Hello

On our kernelci lab, each qemu worker pass an healtcheck job each day and after 
each job failure, so it is heavily used.
The healtcheck job is a Linux boot with a stable release.

Since we upgraded our worker to buster, the qemu x86_64 healthcheck randomly 
panic with:
<6>[    0.001000] APIC: Switch to symmetric I/O mode setup
<6>[    0.001000] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
<3>[    0.005000] ..MP-BIOS bug: 8254 timer not connected to IO-APIC
<6>[    0.005000] ...trying to set up timer (IRQ0) through the 8259A ...
<6>[    0.005000] ..... (found apic 0 pin 2) ...
<6>[    0.009000] ....... failed.
<6>[    0.009000] ...trying to set up timer as Virtual Wire IRQ...
<6>[    0.009000] ..... failed.
<6>[    0.009000] ...trying to set up timer as ExtINT IRQ...
<6>[    0.009000] ..... failed :(.
<0>[    0.009000] 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.
<4>[    0.009000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.4.23 #1
<4>[    0.009000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
1.12.0-1 04/01/2014
<4>[    0.009000] Call Trace:
<4>[    0.009000]  dump_stack+0x50/0x70
<4>[    0.009000]  panic+0xf6/0x2b7
<4>[    0.009000]  setup_IO_APIC+0x7c3/0x81c
<4>[    0.009000]  ? clear_IO_APIC_pin+0xb3/0x100
<4>[    0.009000]  x86_late_time_init+0x1b/0x20
<4>[    0.009000]  start_kernel+0x429/0x4e2
<4>[    0.009000]  secondary_startup_64+0xa4/0xb0

The qemu is called with:
/usr/bin/qemu-system-x86_64 -cpu host -enable-kvm -nographic -net 
nic,model=virtio,macaddr=52:54:00:12:34:58 -net user -m 512 -monitor none 
-kernel 
/var/lib/lava/dispatcher/tmp/741722/deployimages-xl6ogak_/kernel/bzImage 
-append "console=ttyS0,115200 root=/dev/ram0 debug verbose 
console_msg_format=syslog" -initrd 
/var/lib/lava/dispatcher/tmp/741722/deployimages-xl6ogak_/ramdisk/rootfs.cpio.gz
 -drive 
format=qcow2,file=/var/lib/lava/dispatcher/tmp/741722/apply-overlay-guest-sfn3zqna/lava-guest.qcow2,media=disk,if=ide,id=lavatest

We have tried to upgrade the Linux version from 5.0.21 to 5.4.23 without any 
change.
Only our buster worker fail like this, no problem with stretch.

We believing that only buster's qemu was failling since my other lab (gentoo 
with qemu 4.2) never failed.
This was until yesterday, were I hit the same problem on this gentoo lab.

After some test I found the source of this kernel panic, the host is loaded and 
qemu run "slower".
Simply renicing all qemu removed this behavour.

So now what can I do ?
Appart renicing qemu process, does something could be done ?

Thanks
Regards