linux-kernel - Re: [PATCH v2] KVM: Extend MAX_IRQ

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Message-ID: <CANRm+Cyoufp89=orEMBst3uwLAn6gfuNkh4g80upM52NOLpDFw@mail.gmail.com>
Date:   Mon, 23 Apr 2018 19:56:10 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Tonny Lu <tonnylu@...cent.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Subject: Re: [PATCH v2] KVM: Extend MAX_IRQ_ROUTES to 4096 for all archs

2018-04-23 19:50 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
>
>
> On 04/21/2018 02:38 AM, Wanpeng Li wrote:
>> 2018-04-20 22:21 GMT+08:00 Cornelia Huck <cohuck@...hat.com>:
>>> On Fri, 20 Apr 2018 21:51:13 +0800
>>> Wanpeng Li <kernellwp@...il.com> wrote:
>>>
>>>> 2018-04-20 15:15 GMT+08:00 Cornelia Huck <cohuck@...hat.com>:
>>>>> On Thu, 19 Apr 2018 17:47:28 -0700
>>>>> Wanpeng Li <kernellwp@...il.com> wrote:
>>>>>
>>>>>> From: Wanpeng Li <wanpengli@...cent.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@...hat.com>
>>>>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>>>>> Cc: Tonny Lu <tonnylu@...cent.com>
>>>>>> Cc: Cornelia Huck <cohuck@...hat.com>
>>>>>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>>>>>> Signed-off-by: Tonny Lu <tonnylu@...cent.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.

Thanks Christian. Paolo, could you pick this one w/ "/* might need
extension/rework in the future */ instead of
the FIXME" change or do you need I to send out a new version? :)

Regards,
Wanpeng Li