On Tue, Sep 6, 2022 at 12:46 PM Daniel Vetter daniel@ffwll.ch wrote:

On Mon, Aug 01, 2022 at 10:04:55AM -0700, Rob Clark wrote:

...

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). 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

v2. fix compiler warning

I think I bikeshedded this on irc already, here for the record too.

You should look at v3 (which I confusingly titled as v2, sry):

https://patchwork.freedesktop.org/patch/497799/?series=106847&rev=3

• this wont work for buffers which do change the mapping when they move (ttm can do that). And cros does make noises about discrete gpus I've heard, this matters even for you :-)

Correct, in v3 you could use DMA_BUF_MAP_INCOHERENT for this case (or we could add additional enum values.. DMA_BUF_MAP_IDK or whatever)

re: dgpu, I guess those will be pass-thru so not so relevant for this issue

• I'm pretty sure this will put is even more onto the nasty people list that dma-api folks maintain, especially with passing this all to userspace
• follow_pte() can figure this out internally in the kernel and kvm is already using this, and I think doing this all internally with mmu notifier and what not to make sure it all stays in sync is the right approach. So your kvm/whatever combo should be able to figure out wth it's supposed to be doing.

This doesn't help, because the VMM is in userspace.. it is the VMM which needs this information.

I think if you make this a virtio special case like we've done with the magic uuid stuff, then that would make sense. Making it a full dma-buf interface doesn't imo.

IMHO we can consider this (at least in case of v3) as a virtio special, at least in the sense that it is opt-in for the exporting driver, and exporting drivers are free to not play along

BR, -R

Cheers, Daniel

...

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..87c52f080274 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, 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, (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

2.36.1

---

Linaro-mm-sig mailing list -- linaro-mm-sig@lists.linaro.org To unsubscribe send an email to linaro-mm-sig-leave@lists.linaro.org

-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch