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Subject: Re: [PATCH v4 06/13] KVM: Move WARN on invalid memslot index to
 update_memslots()
To:     "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc:     Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
References: <cover.1628871411.git.maciej.szmigiero@oracle.com>
 <8db0f1d1901768b5de1417caa425e62d1118e5e8.1628871413.git.maciej.szmigiero@oracle.com>
 <957c6b3d-9621-a5a5-418c-f61f87a32ee0@redhat.com>
 <fa71d652-8b7f-e0d7-5617-8958e3e78f6e@maciej.szmigiero.name>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <633bf50b-5de4-1e76-736c-067d10bf92b3@redhat.com>
Date:   Fri, 20 Aug 2021 11:27:42 +0200
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On 18.08.21 23:43, Maciej S. Szmigiero wrote:
> On 18.08.2021 16:35, David Hildenbrand wrote:
>> On 13.08.21 21:33, Maciej S. Szmigiero wrote:
>>> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>>>
>>> Since kvm_memslot_move_forward() can theoretically return a negative
>>> memslot index even when kvm_memslot_move_backward() returned a positive one
>>> (and so did not WARN) let's just move the warning to the common code.
>>>
>>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>>> ---
>>>    virt/kvm/kvm_main.c | 6 ++++--
>>>    1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index 03ef42d2e421..7000efff1425 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -1293,8 +1293,7 @@ static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
>>>        struct kvm_memory_slot *mslots = slots->memslots;
>>>        int i;
>>> -    if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
>>> -        WARN_ON_ONCE(!slots->used_slots))
>>> +    if (slots->id_to_index[memslot->id] == -1 || !slots->used_slots)
>>>            return -1;
>>>        /*
>>> @@ -1398,6 +1397,9 @@ static void update_memslots(struct kvm_memslots *slots,
>>>                i = kvm_memslot_move_backward(slots, memslot);
>>>            i = kvm_memslot_move_forward(slots, memslot, i);
>>> +        if (WARN_ON_ONCE(i < 0))
>>> +            return;
>>> +
>>>            /*
>>>             * Copy the memslot to its new position in memslots and update
>>>             * its index accordingly.
>>>
>>
>>
>> Note that WARN_ON_* is frowned upon, because it can result in crashes with panic_on_warn enabled, which is what some distributions do enable.
>>
>> We tend to work around that by using pr_warn()/pr_warn_once(), avoiding eventually crashing the system when there is a way to continue.
>>
> 
> This patch uses WARN_ON_ONCE because:
> 1) It was used in the old code and the patch merely moves the check
> from kvm_memslot_move_backward() to its caller,
> 
> 2) This chunk of code is wholly replaced by patch 11 from this series
> anyway ("Keep memslots in tree-based structures instead of array-based ones").

Okay, that makes sense then, thanks!

-- 
Thanks,

David / dhildenb