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SubjectRe: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Add ioctl to query mmap info
From
Am 29.07.22 um 19:07 schrieb Rob Clark:
> From: Rob Clark <robdclark@chromium.org>
>
> This is a fairly narrowly focused interface, providing a way for a VMM
> in userspace to tell the guest kernel what pgprot settings to use when
> mapping a buffer to guest userspace.
>
> For buffers that get mapped into guest userspace, virglrenderer returns
> a dma-buf fd to the VMM (crosvm or qemu).

Wow, wait a second. Who is giving whom the DMA-buf fd here?

My last status was that this design was illegal and couldn't be
implemented because it requires internal knowledge only the exporting
driver can have.

@Daniel has anything changed on that is or my status still valid?

Regards,
Christian.

> In addition to mapping the
> pages into the guest VM, it needs to report to drm/virtio in the guest
> the cache settings to use for guest userspace. In particular, on some
> architectures, creating aliased mappings with different cache attributes
> is frowned upon, so it is important that the guest mappings have the
> same cache attributes as any potential host mappings.
>
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> drivers/dma-buf/dma-buf.c | 26 ++++++++++++++++++++++++++
> include/linux/dma-buf.h | 7 +++++++
> include/uapi/linux/dma-buf.h | 28 ++++++++++++++++++++++++++++
> 3 files changed, 61 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 32f55640890c..d02d6c2a3b49 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -326,6 +326,29 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> return 0;
> }
>
> +static long dma_buf_info(struct dma_buf *dmabuf, const void __user *uarg)
> +{
> + struct dma_buf_info arg;
> +
> + if (copy_from_user(&arg, uarg, sizeof(arg)))
> + return -EFAULT;
> +
> + switch (arg.param) {
> + case DMA_BUF_INFO_VM_PROT:
> + if (!dmabuf->ops->mmap_info)
> + return -ENOSYS;
> + arg.value = dmabuf->ops->mmap_info(dmabuf);
> + break;
> + default:
> + return -EINVAL;
> + }
> +
> + if (copy_to_user(uarg, &arg, sizeof(arg)))
> + return -EFAULT;
> +
> + return 0;
> +}
> +
> static long dma_buf_ioctl(struct file *file,
> unsigned int cmd, unsigned long arg)
> {
> @@ -369,6 +392,9 @@ static long dma_buf_ioctl(struct file *file,
> case DMA_BUF_SET_NAME_B:
> return dma_buf_set_name(dmabuf, (const char __user *)arg);
>
> + case DMA_BUF_IOCTL_INFO:
> + return dma_buf_info(dmabuf, (const void __user *)arg);
> +
> default:
> return -ENOTTY;
> }
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 71731796c8c3..6f4de64a5937 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -283,6 +283,13 @@ struct dma_buf_ops {
> */
> int (*mmap)(struct dma_buf *, struct vm_area_struct *vma);
>
> + /**
> + * @mmap_info:
> + *
> + * Return mmapping info for the buffer. See DMA_BUF_INFO_VM_PROT.
> + */
> + int (*mmap_info)(struct dma_buf *);
> +
> int (*vmap)(struct dma_buf *dmabuf, struct iosys_map *map);
> void (*vunmap)(struct dma_buf *dmabuf, struct iosys_map *map);
> };
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index b1523cb8ab30..a41adac0f46a 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -85,6 +85,32 @@ struct dma_buf_sync {
>
> #define DMA_BUF_NAME_LEN 32
>
> +
> +/**
> + * struct dma_buf_info - Query info about the buffer.
> + */
> +struct dma_buf_info {
> +
> +#define DMA_BUF_INFO_VM_PROT 1
> +# define DMA_BUF_VM_PROT_WC 0
> +# define DMA_BUF_VM_PROT_CACHED 1
> +
> + /**
> + * @param: Which param to query
> + *
> + * DMA_BUF_INFO_BM_PROT:
> + * Query the access permissions of userspace mmap's of this buffer.
> + * Returns one of DMA_BUF_VM_PROT_x
> + */
> + __u32 param;
> + __u32 pad;
> +
> + /**
> + * @value: Return value of the query.
> + */
> + __u64 value;
> +};
> +
> #define DMA_BUF_BASE 'b'
> #define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>
> @@ -95,4 +121,6 @@ struct dma_buf_sync {
> #define DMA_BUF_SET_NAME_A _IOW(DMA_BUF_BASE, 1, __u32)
> #define DMA_BUF_SET_NAME_B _IOW(DMA_BUF_BASE, 1, __u64)
>
> +#define DMA_BUF_IOCTL_INFO _IOWR(DMA_BUF_BASE, 2, struct dma_buf_info)
> +
> #endif

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