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Message-ID: <973de2f8-75e4-d4c7-a13a-c541a6cf7c77@amd.com>
Date: Sun, 7 Aug 2022 19:14:43 +0200
From: Christian König <christian.koenig@....com>
To: Rob Clark <robdclark@...il.com>,
Christian König <ckoenig.leichtzumerken@...il.com>
Cc: dri-devel@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
freedreno@...ts.freedesktop.org,
Rob Clark <robdclark@...omium.org>,
Sumit Semwal <sumit.semwal@...aro.org>,
Jérôme Pouiller <jerome.pouiller@...abs.com>,
"open list:DMA BUFFER SHARING FRAMEWORK"
<linux-media@...r.kernel.org>,
"moderated list:DMA BUFFER SHARING FRAMEWORK"
<linaro-mm-sig@...ts.linaro.org>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH 1/3] dma-buf: Add ioctl to query mmap info
Am 07.08.22 um 19:02 schrieb Rob Clark:
> On Sun, Aug 7, 2022 at 9:09 AM Christian König
> <ckoenig.leichtzumerken@...il.com> wrote:
>> Am 29.07.22 um 19:07 schrieb Rob Clark:
>>> From: Rob Clark <robdclark@...omium.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@...omium.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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