linux-kernel - Re: [PATCH V7 4/5] virtio-scsi: introduce multiqueue support

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Message-ID: <51505ED2.70208@redhat.com>
Date:	Mon, 25 Mar 2013 15:27:30 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bart Van Assche <bvanassche@....org>
CC:	Wanlong Gao <gaowanlong@...fujitsu.com>,
	linux-scsi@...r.kernel.org, kvm@...r.kernel.org, mst@...hat.com,
	linux-kernel@...r.kernel.org, JBottomley@...allels.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH V7 4/5] virtio-scsi: introduce multiqueue support

Il 25/03/2013 08:25, Bart Van Assche ha scritto:
>>
>> +        queue_num = smp_processor_id();
>> +        while (unlikely(queue_num >= vscsi->num_queues))
>> +            queue_num -= vscsi->num_queues;
>> +
>> +        tgt->req_vq = vq = &vscsi->req_vqs[queue_num];
>> +    }
>> +
>> +    spin_unlock_irqrestore(&tgt->tgt_lock, flags);
>> +    return vq;
>> +}
> 
> Is there any reason why the smp_processor_id() % vscsi->num_queues
> computation in virtscsi_pick_vq() has been implemented as a loop instead
> of as an arithmetic operation ? If so, I would appreciate it if that
> could be explained in a comment.

The expectation is that vscsi->num_queues is the same as the number of
CPUs in the virtual machine; see patch 5 too.

Paolo
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