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

["Christian König" <christian.koenig@amd.com>]

Am 07.08.22 um 19:02 schrieb Rob Clark:
> On Sun, Aug 7, 2022 at 9:09 AM Christian König
> <ckoenig.leichtzumerken@gmail.com> wrote:
>> 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?
> Not sure I understand the question.. the dma-buf fd could come from
> EGL_MESA_image_dma_buf_export, gbm, or similar.
>
>> 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.
> This ioctl provides that information from the exporting driver so that
> a VMM doesn't have to make assumptions ;-)

And exactly that was NAKed the last time it came up. Only the exporting 
driver is allowed to mmap() the DMA-buf into the guest.

This way you also don't need to transport any caching information anywhere.

> Currently crosvm assumes if (drivername == "i915") then it is a cached
> mapping, otherwise it is wc.  I'm trying to find a way to fix this.
> Suggestions welcome, but because of how mapping to a guest VM works, a
> VMM is a somewhat special case where this information is needed in
> userspace.

Ok that leaves me completely puzzled. How does that work in the first place?

In other words how does the mapping into the guest page tables happen?

Regards,
Christian.

>
> BR,
> -R
>
>> @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