Re: [Linaro-mm-sig] [PATCH v2 1/3] dma-buf: Add ioctl to query mmap info

[Rob Clark]

On Tue, Sep 6, 2022 at 12:46 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Mon, Aug 01, 2022 at 10:04:55AM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxxxxx
> > To unsubscribe send an email to linaro-mm-sig-leave@xxxxxxxxxxxxxxxx
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch