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

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Date:   Fri, 21 Jan 2022 10:53:09 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup.patel@....com, aou@...s.berkeley.edu, atish.patra@....com,
        bp@...en8.de, catalin.marinas@....com, chenhuacai@...nel.org,
        dave.hansen@...ux.intel.com, frankja@...ux.ibm.com,
        frederic@...nel.org, gor@...ux.ibm.com, hca@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        luto@...nel.org, maz@...nel.org, mingo@...hat.com,
        nsaenzju@...hat.com, palmer@...belt.com, paulmck@...nel.org,
        paul.walmsley@...ive.com, peterz@...radead.org, seanjc@...gle.com,
        suzuki.poulose@....com, svens@...ux.ibm.com, tglx@...utronix.de,
        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>
Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs

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.