Re: [PATCH 1/2] virtio_ring: Do not call dma_map_page if sg is already mapped.

From: Wendy Liang <sunnyliangjy@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Wendy Liang <wendy.liang@xilinx.com>,
	jasowang@redhat.com, virtualization@lists.linux-foundation.org,
	Ohad Ben-Cohen <ohad@wizery.com>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	linux-remoteproc@vger.kernel.org, Wendy Liang <jliang@xilinx.com>
Subject: Re: [PATCH 1/2] virtio_ring: Do not call dma_map_page if sg is already mapped.
Date: Fri, 9 Dec 2016 10:19:05 -0800	[thread overview]
Message-ID: <CAA07jV8C6FqN8o2_Os7RK3BWn6Ld0ccFJ7Cj1evV8Bw-EA5P6A@mail.gmail.com> (raw)
In-Reply-To: <20161208184441-mutt-send-email-mst@kernel.org>

HI Michael,

On Thu, Dec 8, 2016 at 8:46 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Wed, Dec 07, 2016 at 10:59:12PM -0800, Wendy Liang wrote:
>> If sg is already dma mapped, do not call dma_map_page() in
>> vring_map_one_sg().
>>
>> In case of rpmsg, rpmsg uses dma_alloc_coherent() to allocate
>> memory to share with the remote. There is no pages setup
>> in dma_alloc_coherent().
>>
>> In this case, we cannot convert the virtual address back to the
>> physical address. In this case, we can setup the sg_dma_addr to
>> store the DMA address, and also mark the sg is already mapped.
>>
>> In the vring, we can detect if the address is already mapped
>> by checking the sg_dma_addr. If yes, do not call dma_map_page().
>>
>> Signed-off-by: Wendy Liang <jliang@xilinx.com>
>> ---
>>  drivers/virtio/virtio_ring.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 489bfc6..9793e1f 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -180,6 +180,12 @@ static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
>>       if (!vring_use_dma_api(vq->vq.vdev))
>>               return (dma_addr_t)sg_phys(sg);
>>
>> +     /* If the sg is already mapped, return the DMA address */
>
> How come we even reach this code for rpmsg?
> Does vring_use_dma_api return true for rpmsg?
I used vdev feature bit VIRTIO_F_IOMMU_PLATFORM.

>
>> +     if (sg_dma_address(sg)) {
>> +             sg->length = sg_dma_len(sg);
>> +             return sg_dma_address(sg);
>> +     }
>> +
>
> Is there a rule that says 0 is not a valid address?
OK, i see. Is it better to check the sg_dma_len() instead?

However, I just noticed yesterday, there is another patch to
virtio_rpmsg_bus posted to linux-remoteproc mailing list trying to
solve the same issue:
[PATCH v1 2/6] rpmsg: virtio_rpmsg_bus: fix sg_set_buf() when addr is
not a valid kernel address. he didn't use sg_dma_address. And doesn't
require to update the virtio_ring implementation.

>
>>       /*
>>        * We can't use dma_map_sg, because we don't use scatterlists in
>>        * the way it expects (we don't guarantee that the scatterlist
>> --
>> 1.9.1
> --
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