linux-kernel - Re: [PATCH 4/5] virtio: Introduce virtio_max_dma

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Date:   Wed, 30 Jan 2019 15:10:29 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Joerg Roedel <joro@...tes.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Christoph Hellwig <hch@....de>
CC:     Jens Axboe <axboe@...nel.dk>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "jfehlig@...e.com" <jfehlig@...e.com>,
        "Grimm, Jon" <Jon.Grimm@....com>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        "jroedel@...e.de" <jroedel@...e.de>
Subject: Re: [PATCH 4/5] virtio: Introduce virtio_max_dma_size()

On 1/29/19 2:43 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> This function returns the maximum segment size for a single
> dma transaction of a virtio device. The possible limit comes
> from the SWIOTLB implementation in the Linux kernel, that
> has an upper limit of (currently) 256kb of contiguous
> memory it can map. Other DMA-API implementations might also
> have limits.
> 
> Use the new dma_max_mapping_size() function to determine the
> maximum mapping size when DMA-API is in use for virtio.
> 
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  drivers/virtio/virtio_ring.c | 10 ++++++++++
>  include/linux/virtio.h       |  2 ++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index cd7e755484e3..9ca3fe6af9fa 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -266,6 +266,16 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
>  	return false;
>  }
>  
> +size_t virtio_max_dma_size(struct virtio_device *vdev)
> +{
> +	size_t max_segment_size = SIZE_MAX;
> +
> +	if (vring_use_dma_api(vdev))
> +		max_segment_size = dma_max_mapping_size(&vdev->dev);
> +
> +	return max_segment_size;
> +}

When I build with these patches and with the virtio devices as modules I
get a build failure. Looks like this needs an EXPORT_SYMBOL_GPL().

Thanks,
Tom

> +
>  static void *vring_alloc_queue(struct virtio_device *vdev, size_t size,
>  			      dma_addr_t *dma_handle, gfp_t flag)
>  {
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index fa1b5da2804e..673fe3ef3607 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -157,6 +157,8 @@ int virtio_device_freeze(struct virtio_device *dev);
>  int virtio_device_restore(struct virtio_device *dev);
>  #endif
>  
> +size_t virtio_max_dma_size(struct virtio_device *vdev);
> +
>  #define virtio_device_for_each_vq(vdev, vq) \
>  	list_for_each_entry(vq, &vdev->vqs, list)
>  
>