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Subject: Re: [PATCH v2] KVM: Extend MAX_IRQ_ROUTES to 4096 for all archs
To:     Wanpeng Li <kernellwp@gmail.com>, Cornelia Huck <cohuck@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Tonny Lu <tonnylu@tencent.com>,
        Janosch Frank <frankja@linux.ibm.com>
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 <CANRm+CwQB_EBK_GSF-Nrm6kfpQoNZJmg+N382B+d4YVNjj_gOA@mail.gmail.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Mon, 23 Apr 2018 13:50:48 +0200
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On 04/21/2018 02:38 AM, Wanpeng Li wrote:
> 2018-04-20 22:21 GMT+08:00 Cornelia Huck <cohuck@redhat.com>:
>> On Fri, 20 Apr 2018 21:51:13 +0800
>> Wanpeng Li <kernellwp@gmail.com> wrote:
>>
>>> 2018-04-20 15:15 GMT+08:00 Cornelia Huck <cohuck@redhat.com>:
>>>> On Thu, 19 Apr 2018 17:47:28 -0700
>>>> Wanpeng Li <kernellwp@gmail.com> wrote:
>>>>
>>>>> From: Wanpeng Li <wanpengli@tencent.com>
>>>>>
>>>>> Our virtual machines make use of device assignment by configuring
>>>>> 12 NVMe disks for high I/O performance. Each NVMe device has 129
>>>>> MSI-X Table entries:
>>>>> Capabilities: [50] MSI-X: Enable+ Count=129 Masked-Vector table: BAR=0 offset=00002000
>>>>> The windows virtual machines fail to boot since they will map the number of
>>>>> MSI-table entries that the NVMe hardware reported to the bus to msi routing
>>>>> table, this will exceed the 1024. This patch extends MAX_IRQ_ROUTES to 4096
>>>>> for all archs, in the future this might be extended again if needed.
>>>>>
>>>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>>>> Cc: Tonny Lu <tonnylu@tencent.com>
>>>>> Cc: Cornelia Huck <cohuck@redhat.com>
>>>>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>>>>> Signed-off-by: Tonny Lu <tonnylu@tencent.com>
>>>>> ---
>>>>> v1 -> v2:
>>>>>  * extend MAX_IRQ_ROUTES to 4096 for all archs
>>>>>
>>>>>  include/linux/kvm_host.h | 6 ------
>>>>>  1 file changed, 6 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>>>> index 6930c63..0a5c299 100644
>>>>> --- a/include/linux/kvm_host.h
>>>>> +++ b/include/linux/kvm_host.h
>>>>> @@ -1045,13 +1045,7 @@ static inline int mmu_notifier_retry(struct kvm *kvm, unsigned long mmu_seq)
>>>>>
>>>>>  #ifdef CONFIG_HAVE_KVM_IRQ_ROUTING
>>>>>
>>>>> -#ifdef CONFIG_S390
>>>>>  #define KVM_MAX_IRQ_ROUTES 4096 //FIXME: we can have more than that...
>>>>
>>>> What about /* might need extension/rework in the future */ instead of
>>>> the FIXME?
>>>
>>> Yeah, I guess the maintainers can help to fix it when applying. :)
>>>
>>>>
>>>> As far as I understand, 4096 should cover most architectures and the
>>>> sane end of s390 configurations, but will not be enough at the scarier
>>>> end of s390. (I'm not sure how much it matters in practice.)
>>>>
>>>> Do we want to make this a tuneable in the future? Do some kind of
>>>> dynamic allocation? Not sure whether it is worth the trouble.
>>>
>>> I think keep as it is currently.
>>
>> My main question here is how long this is enough... the number of
>> virtqueues per device is up to 1K from the initial 64, which makes it
>> possible to hit the 4K limit with fewer virtio devices than before (on
>> s390, each virtqueue uses a routing table entry). OTOH, we don't want
>> giant tables everywhere just to accommodate s390.
> 
> I suspect there is no real scenario to futher extend for s390 since no
> guys report.
> 
>> If the s390 maintainers tell me that nobody is doing the really insane
>> stuff, I'm happy as well :)
> 
> Christian, any thoughts?

For now this patch is a no-op for s390 so as long as nobody complains today we are good.
If it turns out to be "not enough" we can then add a configurable number or whatever.