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Date:   Fri, 21 Jan 2022 10:53:09 +0100
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Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs
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From:   Christian Borntraeger <borntraeger@linux.ibm.com>
To:     Mark Rutland <mark.rutland@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        aleksandar.qemu.devel@gmail.com, alexandru.elisei@arm.com,
        anup.patel@wdc.com, aou@eecs.berkeley.edu, atish.patra@wdc.com,
        bp@alien8.de, catalin.marinas@arm.com, chenhuacai@kernel.org,
        dave.hansen@linux.intel.com, frankja@linux.ibm.com,
        frederic@kernel.org, gor@linux.ibm.com, hca@linux.ibm.com,
        james.morse@arm.com, jmattson@google.com, joro@8bytes.org,
        luto@kernel.org, maz@kernel.org, mingo@redhat.com,
        nsaenzju@redhat.com, palmer@dabbelt.com, paulmck@kernel.org,
        paul.walmsley@sifive.com, peterz@infradead.org, seanjc@google.com,
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        wanpengli@tencent.com, will@kernel.org,
        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>
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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.