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Message-ID: <CAF6AEGvUqoCegMZY-h9O6_vc1i=gD-TPHnFyvCjDA558Xn_5dw@mail.gmail.com>
Date: Wed, 7 Sep 2022 00:57:00 -0700
From: Rob Clark <robdclark@...il.com>
To: Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org,
freedreno@...ts.freedesktop.org,
Rob Clark <robdclark@...omium.org>,
Sumit Semwal <sumit.semwal@...aro.org>,
Christian König <christian.koenig@....com>,
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 v2 1/3] dma-buf: Add ioctl to query mmap info
On Tue, Sep 6, 2022 at 12:46 PM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Mon, Aug 01, 2022 at 10:04:55AM -0700, Rob Clark wrote:
> > 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). 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>
> > ---
> > 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@...ts.linaro.org
> > To unsubscribe send an email to linaro-mm-sig-leave@...ts.linaro.org
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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