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

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Date:   Fri, 11 Aug 2017 16:09:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        "Richard W.M. Jones" <rjones@...hat.com>
Cc:     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 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?  It makes no sense if indirect
descriptors are available, especially because...

> Either tell linux to avoid s/g lists that are too long, or
> simply fail request if this happens, or refuse to attach driver to device.
> 
> Later option would look something like this within probe:
> 
>         for (i = VIRTIO_SCSI_VQ_BASE; i < num_vqs; i++)
> 		if (vqs[i]->num < MAX_SG_USED_BY_LINUX)
> 			goto err;
> 
> 
> I don't know what's MAX_SG_USED_BY_LINUX though.
> 

... 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).

Paolo