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Date:   Mon, 23 Apr 2018 13:57:08 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?UTF-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Tonny Lu <tonnylu@tencent.com>,
        Janosch Frank <frankja@linux.ibm.com>
Subject: Re: [PATCH v2] KVM: Extend MAX_IRQ_ROUTES to 4096 for all archs
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On Mon, 23 Apr 2018 13:50:48 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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. 

OK, then let's deal with the problem once it shows up.

With the comment changed as suggested above,

Reviewed-by: Cornelia Huck <cohuck@redhat.com>