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Date:   Wed, 23 Jan 2019 22:31:39 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     "Michael S . Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        virtualization@lists.linux-foundation.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        iommu@lists.linux-foundation.org, jfehlig@suse.com,
        jon.grimm@amd.com, brijesh.singh@amd.com, jroedel@suse.de
Subject: Re: [PATCH 5/5] virtio-blk: Consider virtio_max_dma_size() for
 maximum segment size
Message-ID: <20190123213139.GD9032@lst.de>
References: <20190123163049.24863-1-joro@8bytes.org> <20190123163049.24863-6-joro@8bytes.org>
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On Wed, Jan 23, 2019 at 05:30:49PM +0100, Joerg Roedel wrote:
> +	max_size = virtio_max_dma_size(vdev);
> +
>  	/* Host can optionally specify maximum segment size and number of
>  	 * segments. */
>  	err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
>  				   struct virtio_blk_config, size_max, &v);
>  	if (!err)
> -		blk_queue_max_segment_size(q, v);
> -	else
> -		blk_queue_max_segment_size(q, -1U);
> +		max_size = min(max_size, v);
> +
> +	blk_queue_max_segment_size(q, max_size);

I wonder if we should just move the dma max segment size check
into blk_queue_max_segment_size so that all block drivers benefit
from it.  Even if not I think at least the SCSI midlayer should
be updated to support it.

Btw, I wonder why virtio-scsi sticks to the default segment size,
unlike virtio-blk.