Re: [PATCH v2] KVM: s390: fix gisa destroy operation might lead to cpu stalls

[Michael Mueller <mimu@linux.ibm.com>]

On 24.08.23 21:17, Matthew Rosato wrote:
> On 8/24/23 9:09 AM, Michael Mueller wrote:
>> A GISA cannot be destroyed as long it is linked in the GIB alert list
>> as this would breake the alert list. Just waiting for its removal from
> 
> Hi Michael,
> 
> Nit: s/breake/break/
> 
>> the list triggered by another vm is not sufficient as it might be the
>> only vm. The below shown cpu stall situation might occur when GIB alerts
>> are delayed and is fixed by calling process_gib_alert_list() instead of
>> waiting.
>>
>> At this time the vcpus of the vm are already destroyed and thus
>> no vcpu can be kicked to enter the SIE again if for some reason an
>> interrupt is pending for that vm.
>>
>> Additianally the IAM restore value ist set to 0x00 if that was not the
> 
> Nits: s/Additianally/Additionally/  as well as s/ist/is/
> 

Thanks a lot, Matt. I will address of course all these typos ;)

>> case. That would be a bug introduced by incomplete device de-registration,
>> i.e. missing kvm_s390_gisc_unregister() call.
> If this implies a bug, maybe it should be a WARN_ON instead of a KVM_EVENT?  Because if we missed a call to kvm_s390_gisc_unregister() then we're also leaking refcounts (one for each gisc that we didn't unregister).

I was thinking of a WARN_ON() as well and will most probaly add it 
because it is much better visible.

> 
>>
>> Setting this value guarantees that late interrupts don't bring the GISA
>> back into the alert list.
> 
> Just to make sure I understand -- The idea is that once you set the alert mask to 0x00 then it should be impossible for millicode to deliver further alerts associated with this gisa right?  Thus making it OK to do one last process_gib_alert_list() after that point in time.
> 
> But I guess my question is: will millicode actually see this gi->alert.mask change soon enough to prevent further alerts?  Don't you need to also cmpxchg the mask update into the contents of kvm_s390_gisa (via gisa_set_iam?) 

It is not the IAM directly that I set to 0x00 but gi->alert.mask. It is 
used the restore the IAM in the gisa by means of 
gisa_get_ipm_or_restore_iam() under cmpxchg() conditions which is called 
by process_gib_alert_list() and the hr_timer function gisa_vcpu_kicker() 
that it triggers. When the gisa is in the alert list, the IAM is always 
0x00. It's set by millicode. I just need to ensure that it is not 
changed to anything else.

in order to ensure an alert can't still be delivered some time after you 
check gisa_in_alert_list(gi->origin)?  That matches up with what is done 
per-gisc in kvm_s390_gisc_unregister() today.

right

> 
> ...  That said, now that I'm looking closer at kvm_s390_gisc_unregister() and gisa_set_iam():  it seems strange that nobody checks the return code from gisa_set_iam today.  AFAICT, even if the device driver(s) call kvm_s390_gisc_unregister correctly for all associated gisc, if gisa_set_iam manages to return -EBUSY because the gisa is already in the alert list then wouldn't the gisc refcounts be decremented but the relevant alert bit left enabled for that gisc until the next time we call gisa_set_iam or gisa_get_ipm_or_restore_iam?

you are right, that should retried in kvm_s390_gisc_register() and 
kvm_s390_gisc_unregister() until the rc is 0 but that would lead to a 
CPU stall as well under the condition where GAL interrupts are not 
delivered in the host.

> 
> Similar strangeness for kvm_s390_gisc_register() - AFAICT if gisa_set_iam returns -EBUSY then we would increment the gisc refcounts but never actually enable the alert bit for that gisc until the next time we call gisa_set_iam or gisa_get_ipm_or_restore_iam.

I have to think and play around with process_gib_alert_list() being 
called as well in these situations.

BTW the pci and the vfip_ap device drivers currently also ignore the 
return codes of kvm_s390_gisc_unregister().

Thanks a lot for your thoughts!
Michael

> 
> Thanks,
> Matt
> 
>>
>> Both situation can now be observed in the kvm-trace:
>>
>>   00 01692880424:653210 3 - 0004 000003ff80136b58  vm 0x000000008e588000 created by pid 3019
>>   00 01692880472:159783 3 - 0002 000003ff80143c06  vm 0x000000008e588000 has unexpected restore iam 0x02
>>   00 01692880472:159784 3 - 0002 000003ff80143c24  vm 0x000000008e588000 gisa in alert list during destroy
>>   00 01692880472:229846 3 - 0004 000003ff8013319a  vm 0x000000008e588000 destroyed
>>
>> CPU stall caused by kvm_s390_gisa_destroy():
>>
>>   [ 4915.311372] rcu: INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 14-.... } 24533 jiffies s: 5269 root: 0x1/.
>>   [ 4915.311390] rcu: blocking rcu_node structures (internal RCU debug): l=1:0-15:0x4000/.
>>   [ 4915.311394] Task dump for CPU 14:
>>   [ 4915.311395] task:qemu-system-s39 state:R  running task     stack:0     pid:217198 ppid:1      flags:0x00000045
>>   [ 4915.311399] Call Trace:
>>   [ 4915.311401]  [<0000038003a33a10>] 0x38003a33a10
>>   [ 4933.861321] rcu: INFO: rcu_sched self-detected stall on CPU
>>   [ 4933.861332] rcu: 	14-....: (42008 ticks this GP) idle=53f4/1/0x4000000000000000 softirq=61530/61530 fqs=14031
>>   [ 4933.861353] rcu: 	(t=42008 jiffies g=238109 q=100360 ncpus=18)
>>   [ 4933.861357] CPU: 14 PID: 217198 Comm: qemu-system-s39 Not tainted 6.5.0-20230816.rc6.git26.a9d17c5d8813.300.fc38.s390x #1
>>   [ 4933.861360] Hardware name: IBM 8561 T01 703 (LPAR)
>>   [ 4933.861361] Krnl PSW : 0704e00180000000 000003ff804bfc66 (kvm_s390_gisa_destroy+0x3e/0xe0 [kvm])
>>   [ 4933.861414]            R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
>>   [ 4933.861416] Krnl GPRS: 0000000000000000 00000372000000fc 00000002134f8000 000000000d5f5900
>>   [ 4933.861419]            00000002f5ea1d18 00000002f5ea1d18 0000000000000000 0000000000000000
>>   [ 4933.861420]            00000002134fa890 00000002134f8958 000000000d5f5900 00000002134f8000
>>   [ 4933.861422]            000003ffa06acf98 000003ffa06858b0 0000038003a33c20 0000038003a33bc8
>>   [ 4933.861430] Krnl Code: 000003ff804bfc58: ec66002b007e	cij	%r6,0,6,000003ff804bfcae
>>                             000003ff804bfc5e: b904003a		lgr	%r3,%r10
>>                            #000003ff804bfc62: a7f40005		brc	15,000003ff804bfc6c
>>                            >000003ff804bfc66: e330b7300204	lg	%r3,10032(%r11)
>>                             000003ff804bfc6c: 58003000		l	%r0,0(%r3)
>>                             000003ff804bfc70: ec03fffb6076	crj	%r0,%r3,6,000003ff804bfc66
>>                             000003ff804bfc76: e320b7600271	lay	%r2,10080(%r11)
>>                             000003ff804bfc7c: c0e5fffea339	brasl	%r14,000003ff804942ee
>>   [ 4933.861444] Call Trace:
>>   [ 4933.861445]  [<000003ff804bfc66>] kvm_s390_gisa_destroy+0x3e/0xe0 [kvm]
>>   [ 4933.861460] ([<00000002623523de>] free_unref_page+0xee/0x148)
>>   [ 4933.861507]  [<000003ff804aea98>] kvm_arch_destroy_vm+0x50/0x120 [kvm]
>>   [ 4933.861521]  [<000003ff8049d374>] kvm_destroy_vm+0x174/0x288 [kvm]
>>   [ 4933.861532]  [<000003ff8049d4fe>] kvm_vm_release+0x36/0x48 [kvm]
>>   [ 4933.861542]  [<00000002623cd04a>] __fput+0xea/0x2a8
>>   [ 4933.861547]  [<00000002620d5bf8>] task_work_run+0x88/0xf0
>>   [ 4933.861551]  [<00000002620b0aa6>] do_exit+0x2c6/0x528
>>   [ 4933.861556]  [<00000002620b0f00>] do_group_exit+0x40/0xb8
>>   [ 4933.861557]  [<00000002620b0fa6>] __s390x_sys_exit_group+0x2e/0x30
>>   [ 4933.861559]  [<0000000262d481f4>] __do_syscall+0x1d4/0x200
>>   [ 4933.861563]  [<0000000262d59028>] system_call+0x70/0x98
>>   [ 4933.861565] Last Breaking-Event-Address:
>>   [ 4933.861566]  [<0000038003a33b60>] 0x38003a33b60
>>
>> Fixes: 9f30f6216378 ("KVM: s390: add gib_alert_irq_handler()")
>> Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
>> ---
>>   arch/s390/kvm/interrupt.c | 12 ++++++++----
>>   1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
>> index 85e39f472bb4..06890a58d001 100644
>> --- a/arch/s390/kvm/interrupt.c
>> +++ b/arch/s390/kvm/interrupt.c
>> @@ -3216,11 +3216,15 @@ void kvm_s390_gisa_destroy(struct kvm *kvm)
>>   
>>   	if (!gi->origin)
>>   		return;
>> -	if (gi->alert.mask)
>> -		KVM_EVENT(3, "vm 0x%pK has unexpected iam 0x%02x",
>> +	if (gi->alert.mask) {
>> +		KVM_EVENT(3, "vm 0x%pK has unexpected restore iam 0x%02x",
>>   			  kvm, gi->alert.mask);
>> -	while (gisa_in_alert_list(gi->origin))
>> -		cpu_relax();
>> +		gi->alert.mask = 0x00;
>> +	}
>> +	if (gisa_in_alert_list(gi->origin)) {
>> +		KVM_EVENT(3, "vm 0x%pK gisa in alert list during destroy", kvm);
>> +		process_gib_alert_list();
>> +	}
>>   	hrtimer_cancel(&gi->timer);
>>   	gi->origin = NULL;
>>   	VM_EVENT(kvm, 3, "gisa 0x%pK destroyed", gisa);
>