linux-kernel - Re: [PATCH 1/2] virtio: Reduce BUG if total

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Date:   Fri, 11 Aug 2017 20:36:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Richard W.M. Jones" <rjones@...hat.com>, jejb@...ux.vnet.ibm.com,
        martin.petersen@...cle.com, jasowang@...hat.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, hch@....de
Subject: Re: [PATCH 1/2] virtio: Reduce BUG if total_sg > virtqueue size to
 WARN.

On 11/08/2017 19:23, Michael S. Tsirkin wrote:
> On Fri, Aug 11, 2017 at 04:09:26PM +0200, Paolo Bonzini wrote:
>> On 10/08/2017 23:41, Michael S. Tsirkin wrote:
>>>>> Then we probably should fail probe if vq size is too small.
>>>> What does this mean?
>>>
>>> We must prevent driver from submitting s/g lists > vq size to device.
>>
>> What is the rationale for the limit?
> 
> So the host knows what it needs to support.
> 
>> both virtio-blk and virtio-scsi transmit their own value for the
>> maximum sg list size (max_seg in virtio-scsi, seg_max in virtio-blk).
> 
> No other device has it, and it seemed like a good idea to
> limit it generally at the time.
> 
> we can fix the spec to relax the requirement for blk and scsi -
> want to submit a proposal? Alternatively, add a generic field
> for that.

Yes, I can submit a proposal.  blk and scsi are the ones that are most
likely to have very long sg lists.

When I was designing scsi I just copied that field from blk. :)

Paolo