Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 21.01.22 um 15:30 schrieb Mark Rutland:
> On Fri, Jan 21, 2022 at 03:17:01PM +0100, Christian Borntraeger wrote:
>>
>>
>> Am 21.01.22 um 10:53 schrieb Christian Borntraeger:
>>> Am 20.01.22 um 16:14 schrieb Christian Borntraeger:
>>>>
>>>>
>>>> Am 20.01.22 um 13:03 schrieb Mark Rutland:
>>>>> On Thu, Jan 20, 2022 at 12:28:09PM +0100, Paolo Bonzini wrote:
>>>>>> On 1/19/22 20:22, Mark Rutland wrote:
>>>>>>> I wonder, is the s390 guest entry/exit*preemptible*  ?
>>>>>>>
>>>>>>> If a timer IRQ can preempt in the middle of the EQS, we wouldn't balance
>>>>>>> things before a ctx-switch to the idle thread, which would then be able
>>>>>>> to hit this.
>>>>>>>
>>>>>>> I'll need to go audit the other architectures for similar.
>>>>>>
>>>>>> They don't enable interrupts in the entry/exit path so they should be okay.
>>>>>
>>>>> True.
>>>>>
>>>>> So it sounds like for s390 adding an explicit preempt_{disable,enable}() is the
>>>>> right thing to do. I'll add that and explanatory commentary.
>>>>
>>>> That would not be trivial I guess. We do allow (and need) page faults on sie for guest
>>>> demand paging and
>>>>
>>>> this piece of arch/s390/mm/fault.c
>>>>
>>>>          case GMAP_FAULT:
>>>>                   if (faulthandler_disabled() || !mm)
>>>>                           goto out;
>>>>                   break;
>>>>           }
>>>>
>>>> would no longer work since faulthandler_disabled checks for the preempt count.
>>>>
>>>
>>>
>>> Something like this
>>>
>>>
>>> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
>>> index d30f5986fa85..1c7d45346e12 100644
>>> --- a/arch/s390/mm/fault.c
>>> +++ b/arch/s390/mm/fault.c
>>> @@ -385,10 +385,18 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>>>                           return 0;
>>>                   goto out;
>>>           case USER_FAULT:
>>> -       case GMAP_FAULT:
>>>                   if (faulthandler_disabled() || !mm)
>>>                           goto out;
>>>                   break;
>>> +               /*
>>> +                * We know that we interrupted SIE and we are not in a IRQ.
>>> +                * preemption might be disabled thus checking for in_atomic
>>> +                * would result in failures
>>> +                */
>>> +       case GMAP_FAULT:
>>> +               if (pagefault_disabled() || !mm)
>>> +                       goto out;
>>> +               break;
>>>           }
>>>
>>>           perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>>>
>>> seems to work with preemption disabled around sie. Not sure yet if this is correct.
>>
>>
>> No it does not work. scheduling while preemption is disabled.
> 
> Hmm... which exceptions do we expect to take from a guest?
> 
> I wonder if we can handle those more like other architectures by getting those
> to immediately return from the sie64a() call with some status code that we can
> handle outside of the guest_state_{enter,exit}_irqoff() critical section.

We take all kind of page faults (invalid->valid, ro->rw) on the sie instruction and
run that in the context of the pgm_check handler just like for userspace. the pgm_check
handler does a sie_exit (similar to the interrupt handlers) by setting the return IA.
  
> On arm64 in VHE mode, we swap our exception vectors when entering/exiting the
> guest to allow us to do that; I wonder if we could do similar here?