Re: [kvm-unit-tests PATCH 0/2] x86/apic: fix false test_apic_change_mode failures on stalled vCPUs

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 28 Apr 2026 15:35:22 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> test_apic_change_mode sporadically fails in CI on both Intel and AMD
> hosts with errors like:
>   "FAIL: TMCCT should have a non-zero value"
>   "FAIL: TMCCT should be reset to the initial-count"
>   "FAIL: TMCCT should not be reset to TMICT value"
> 
> The root cause is that the APIC timer runs at wall clock time under KVM.
> With the default tmict=0x999999 (~10ms period at 1ns bus cycle).
> 
> A vCPU stall for sufficiently large portion of TMICT leads to false positives
> (reasons could be: host preemption, cross-socket migration, heavy CPU
> contention). It's basically not possible to reliably sample timer values
> while it's running.
> 
> This series adds retry logic with increasing timer periods (10ms, 60ms,
> 700ms) so that transient vCPU stalls don't cause false failures, while
> real bugs still get caught. (most of false failures are handled by 60ms
> timer, and 700ms is one pathological case observed in a week of testing)

looks like this was lost,
ping

> 
> Reproducer (requires 2+ NUMA nodes):
> 
>   stress-ng --cpu 128 --timer 32 --hrtimers 32 --quiet &
>   sleep 2
>   while true; do
>       /usr/libexec/qemu-kvm --no-reboot -nodefaults \
>           -global kvm-pit.lost_tick_policy=discard \
>           -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 \
>           -display none -serial stdio -device pci-testdev \
>           -machine q35 -kernel x86/apic.flat \
>           -smp 1 -cpu qemu64,+x2apic,+tsc-deadline \
>           >> apic_race.log 2>&1 &  
>       QEMU_PID=$!
>       while kill -0 $QEMU_PID 2>/dev/null; do
>           taskset -p -c 0 $QEMU_PID 2>/dev/null
>           sleep 0.001
>           taskset -p -c 1 $QEMU_PID 2>/dev/null
>           sleep 0.001
>       done
>       wait $QEMU_PID 2>/dev/null
>   done
> 
> patches reduce ~4% failure rate (8 FAILs / 216 PASSes in 2 minutes).
> to 0 FAILs over thousands of runs.
> 
> Igor Mammedov (2):
>   x86/apic: separate reporting from actual measurements
>   x86/apic: add retry logic to test_apic_change_mode
> 
>  x86/apic.c | 119 ++++++++++++++++++++++++++++++++---------------------
>  1 file changed, 72 insertions(+), 47 deletions(-)
>