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Date:   Thu, 20 Jan 2022 16:14:13 +0100
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Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs
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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,
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        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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        Anup Patel <anup@brainfault.org>,
        Atish Patra <atishp@atishpatra.org>
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From:   Christian Borntraeger <borntraeger@linux.ibm.com>
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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.