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SubjectRe: [PATCH v2 1/3] dma-buf: Add ioctl to query mmap coherency/cache info
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Am 18.08.22 um 16:25 schrieb Rob Clark:
> On Thu, Aug 18, 2022 at 4:21 AM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 17.08.22 um 15:44 schrieb Rob Clark:
>>> On Wed, Aug 17, 2022 at 2:57 AM Christian König
>>> <christian.koenig@amd.com> wrote:
>>>> [SNIP]
>>>>
>>>> The resulting cache attrs from combination of S1 and S2 translation
>>>> can differ. So ideally we setup the S2 pgtables in guest aligned with
>>>> host userspace mappings
>>>> Well exactly that is not very convincing.
>>>>
>>>> What you want to do is to use one channel for the address and a
>>>> different one for the cache attrs, that's not something I would
>>>> recommend doing in general.
>>> How would that work.. mmap() is the channel for the address, we'd need
>>> to introduce a new syscall that returned additional information?
>> The channel for the address is not mmap(), but rather the page faults.
>> mmap() is just the function setting up that channel.
>>
>> The page faults then insert both the address as well as the caching
>> attributes (at least on x86).
> This is true on arm64 as well, but only in the S1 tables (which I
> would have to assume is the case on x86 as well)
>
>> That we then need to forward the caching attributes manually once more
>> seems really misplaced.
>>
>>>> Instead the client pgtables should be setup in a way so that host can
>>>> overwrite them.
>>> How? That is completely not how VMs work. Even if the host knew
>>> where the pgtables were and somehow magically knew the various guest
>>> userspace VAs, it would be racey.
>> Well you mentioned that the client page tables can be setup in a way
>> that the host page tables determine what caching to use. As far as I can
>> see this is what we should use here.
> On arm64/aarch64, they *can*.. but the system (on some versions of
> armv8) can also be configured to let S2 determine the attributes. And
> apparently there are benefits to this (avoids unnecessary cache
> flushing in the host, AFAIU.) This is the case where we need this new
> api.
>
> IMO it is fine for the exporter to return a value indicating that the
> attributes change dynamically or that S1 attributes must somehow be
> used by the hw. This would at least let the VMM return an error in
> cases where S1 attrs cannot be relied on. But there are enough
> exporters where the cache attrs are static for the life of the buffer.
> So even if you need to return DMA_BUF_MAP_I_DONT_KNOW, maybe that is
> fine (if x86 can always rely on S1 attrs), or at least will let the
> VMM return an error rather than just blindly assuming things will
> work.
>
> But it makes no sense to reject the whole idea just because of some
> exporters (which may not even need this). There is always room to let
> them return a map-info value that describes the situation or
> limitations to the VMM.

Well it does make sense as far as I can see.

This is a very specific workaround for a platform problem which only
matters there, but increases complexity for everybody.

If we don't have any other choice on the problem to work around that I
would say ok we add an ARM specific workaround.

But as long as that's not the case the whole idea is pretty clearly a
NAK from my side.

Regards,
Christian.

>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>> BR,
>>> -R
>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> BR,
>>>>> -R
>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>>>
>>>>>>> BR,
>>>>>>> -R
>>>>>>>
>>>>>>>> If the hardware can't use the caching information from the host CPU page
>>>>>>>> tables directly then that pretty much completely breaks the concept that
>>>>>>>> the exporter is responsible for setting up those page tables.
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Christian.
>>>>>>>>
>>>>>>>>> drivers/dma-buf/dma-buf.c | 63 +++++++++++++++++++++++++++------
>>>>>>>>> include/linux/dma-buf.h | 11 ++++++
>>>>>>>>> include/uapi/linux/dma-buf.h | 68 ++++++++++++++++++++++++++++++++++++
>>>>>>>>> 3 files changed, 132 insertions(+), 10 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>>>>>>>> index 32f55640890c..262c4706f721 100644
>>>>>>>>> --- a/drivers/dma-buf/dma-buf.c
>>>>>>>>> +++ b/drivers/dma-buf/dma-buf.c
>>>>>>>>> @@ -125,6 +125,32 @@ static struct file_system_type dma_buf_fs_type = {
>>>>>>>>> .kill_sb = kill_anon_super,
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> +static int __dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
>>>>>>>>> +{
>>>>>>>>> + int ret;
>>>>>>>>> +
>>>>>>>>> + /* check if buffer supports mmap */
>>>>>>>>> + if (!dmabuf->ops->mmap)
>>>>>>>>> + return -EINVAL;
>>>>>>>>> +
>>>>>>>>> + ret = dmabuf->ops->mmap(dmabuf, vma);
>>>>>>>>> +
>>>>>>>>> + /*
>>>>>>>>> + * If the exporter claims to support coherent access, ensure the
>>>>>>>>> + * pgprot flags match the claim.
>>>>>>>>> + */
>>>>>>>>> + if ((dmabuf->map_info != DMA_BUF_MAP_INCOHERENT) && !ret) {
>>>>>>>>> + pgprot_t wc_prot = pgprot_writecombine(vma->vm_page_prot);
>>>>>>>>> + if (dmabuf->map_info == DMA_BUF_COHERENT_WC) {
>>>>>>>>> + WARN_ON_ONCE(pgprot_val(vma->vm_page_prot) != pgprot_val(wc_prot));
>>>>>>>>> + } else {
>>>>>>>>> + WARN_ON_ONCE(pgprot_val(vma->vm_page_prot) == pgprot_val(wc_prot));
>>>>>>>>> + }
>>>>>>>>> + }
>>>>>>>>> +
>>>>>>>>> + return ret;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
>>>>>>>>> {
>>>>>>>>> struct dma_buf *dmabuf;
>>>>>>>>> @@ -134,16 +160,12 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
>>>>>>>>>
>>>>>>>>> dmabuf = file->private_data;
>>>>>>>>>
>>>>>>>>> - /* check if buffer supports mmap */
>>>>>>>>> - if (!dmabuf->ops->mmap)
>>>>>>>>> - return -EINVAL;
>>>>>>>>> -
>>>>>>>>> /* check for overflowing the buffer's size */
>>>>>>>>> if (vma->vm_pgoff + vma_pages(vma) >
>>>>>>>>> dmabuf->size >> PAGE_SHIFT)
>>>>>>>>> return -EINVAL;
>>>>>>>>>
>>>>>>>>> - return dmabuf->ops->mmap(dmabuf, vma);
>>>>>>>>> + return __dma_buf_mmap(dmabuf, vma);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
>>>>>>>>> @@ -326,6 +348,27 @@ 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_MAP_INFO:
>>>>>>>>> + arg.value = dmabuf->map_info;
>>>>>>>>> + 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 +412,9 @@ 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;
>>>>>>>>> }
>>>>>>>>> @@ -530,6 +576,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
>>>>>>>>> dmabuf->priv = exp_info->priv;
>>>>>>>>> dmabuf->ops = exp_info->ops;
>>>>>>>>> dmabuf->size = exp_info->size;
>>>>>>>>> + dmabuf->map_info = exp_info->map_info;
>>>>>>>>> dmabuf->exp_name = exp_info->exp_name;
>>>>>>>>> dmabuf->owner = exp_info->owner;
>>>>>>>>> spin_lock_init(&dmabuf->name_lock);
>>>>>>>>> @@ -1245,10 +1292,6 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
>>>>>>>>> if (WARN_ON(!dmabuf || !vma))
>>>>>>>>> return -EINVAL;
>>>>>>>>>
>>>>>>>>> - /* check if buffer supports mmap */
>>>>>>>>> - if (!dmabuf->ops->mmap)
>>>>>>>>> - return -EINVAL;
>>>>>>>>> -
>>>>>>>>> /* check for offset overflow */
>>>>>>>>> if (pgoff + vma_pages(vma) < pgoff)
>>>>>>>>> return -EOVERFLOW;
>>>>>>>>> @@ -1262,7 +1305,7 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
>>>>>>>>> vma_set_file(vma, dmabuf->file);
>>>>>>>>> vma->vm_pgoff = pgoff;
>>>>>>>>>
>>>>>>>>> - return dmabuf->ops->mmap(dmabuf, vma);
>>>>>>>>> + return __dma_buf_mmap(dmabuf, vma);
>>>>>>>>> }
>>>>>>>>> EXPORT_SYMBOL_NS_GPL(dma_buf_mmap, DMA_BUF);
>>>>>>>>>
>>>>>>>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>>>>>>>> index 71731796c8c3..37923c8d5c24 100644
>>>>>>>>> --- a/include/linux/dma-buf.h
>>>>>>>>> +++ b/include/linux/dma-buf.h
>>>>>>>>> @@ -23,6 +23,8 @@
>>>>>>>>> #include <linux/dma-fence.h>
>>>>>>>>> #include <linux/wait.h>
>>>>>>>>>
>>>>>>>>> +#include <uapi/linux/dma-buf.h>
>>>>>>>>> +
>>>>>>>>> struct device;
>>>>>>>>> struct dma_buf;
>>>>>>>>> struct dma_buf_attachment;
>>>>>>>>> @@ -307,6 +309,13 @@ struct dma_buf {
>>>>>>>>> */
>>>>>>>>> size_t size;
>>>>>>>>>
>>>>>>>>> + /**
>>>>>>>>> + * @map_info:
>>>>>>>>> + *
>>>>>>>>> + * CPU mapping/coherency information for the buffer.
>>>>>>>>> + */
>>>>>>>>> + enum dma_buf_map_info map_info;
>>>>>>>>> +
>>>>>>>>> /**
>>>>>>>>> * @file:
>>>>>>>>> *
>>>>>>>>> @@ -533,6 +542,7 @@ struct dma_buf_attachment {
>>>>>>>>> * @ops: Attach allocator-defined dma buf ops to the new buffer
>>>>>>>>> * @size: Size of the buffer - invariant over the lifetime of the buffer
>>>>>>>>> * @flags: mode flags for the file
>>>>>>>>> + * @map_info: CPU mapping/coherency information for the buffer
>>>>>>>>> * @resv: reservation-object, NULL to allocate default one
>>>>>>>>> * @priv: Attach private data of allocator to this buffer
>>>>>>>>> *
>>>>>>>>> @@ -545,6 +555,7 @@ struct dma_buf_export_info {
>>>>>>>>> const struct dma_buf_ops *ops;
>>>>>>>>> size_t size;
>>>>>>>>> int flags;
>>>>>>>>> + enum dma_buf_map_info map_info;
>>>>>>>>> struct dma_resv *resv;
>>>>>>>>> void *priv;
>>>>>>>>> };
>>>>>>>>> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
>>>>>>>>> index b1523cb8ab30..07b403ffdb43 100644
>>>>>>>>> --- a/include/uapi/linux/dma-buf.h
>>>>>>>>> +++ b/include/uapi/linux/dma-buf.h
>>>>>>>>> @@ -85,6 +85,72 @@ struct dma_buf_sync {
>>>>>>>>>
>>>>>>>>> #define DMA_BUF_NAME_LEN 32
>>>>>>>>>
>>>>>>>>> +/**
>>>>>>>>> + * enum dma_buf_map_info - CPU mapping info
>>>>>>>>> + *
>>>>>>>>> + * This enum describes coherency of a userspace mapping of the dmabuf.
>>>>>>>>> + *
>>>>>>>>> + * Importing devices should check dma_buf::map_info flag and reject an
>>>>>>>>> + * import if unsupported. For example, if the exporting device uses
>>>>>>>>> + * @DMA_BUF_COHERENT_CACHED but the importing device does not support
>>>>>>>>> + * CPU cache coherency, the dma-buf import should fail.
>>>>>>>>> + */
>>>>>>>>> +enum dma_buf_map_info {
>>>>>>>>> + /**
>>>>>>>>> + * @DMA_BUF_MAP_INCOHERENT: CPU mapping is incoherent.
>>>>>>>>> + *
>>>>>>>>> + * Use of DMA_BUF_IOCTL_SYNC is required for CPU managed coherenency.
>>>>>>>>> + */
>>>>>>>>> + DMA_BUF_MAP_INCOHERENT,
>>>>>>>>> +
>>>>>>>>> + /**
>>>>>>>>> + * @DMA_BUF_COHERENT_WC: CPU mapping is coherent but not cached.
>>>>>>>>> + *
>>>>>>>>> + * A cpu mmap'ing is coherent, and DMA_BUF_IOCTL_SYNC is not required.
>>>>>>>>> + * However fences may be still required for synchronizing access. Ie.
>>>>>>>>> + * coherency can only be relied upon by an explicit-fencing userspace.
>>>>>>>>> + * An implicit-sync userspace must still use DMA_BUF_IOCTL_SYNC.
>>>>>>>>> + *
>>>>>>>>> + * The cpu mapping is writecombine.
>>>>>>>>> + */
>>>>>>>>> + DMA_BUF_COHERENT_WC,
>>>>>>>>> +
>>>>>>>>> + /**
>>>>>>>>> + * @DMA_BUF_COHERENT_CACHED: CPU mapping is coherent and CPU cached.
>>>>>>>>> + *
>>>>>>>>> + * A cpu mmap'ing is coherent, and DMA_BUF_IOCTL_SYNC is not required.
>>>>>>>>> + * However fences may be still required for synchronizing access. Ie.
>>>>>>>>> + * coherency can only be relied upon by an explicit-fencing userspace.
>>>>>>>>> + * An implicit-sync userspace must still use DMA_BUF_IOCTL_SYNC.
>>>>>>>>> + *
>>>>>>>>> + * The cpu mapping is cached.
>>>>>>>>> + */
>>>>>>>>> + DMA_BUF_COHERENT_CACHED,
>>>>>>>>> +};
>>>>>>>>> +
>>>>>>>>> +/**
>>>>>>>>> + * struct dma_buf_info - Query info about the buffer.
>>>>>>>>> + */
>>>>>>>>> +struct dma_buf_info {
>>>>>>>>> +
>>>>>>>>> +#define DMA_BUF_INFO_MAP_INFO 1
>>>>>>>>> +
>>>>>>>>> + /**
>>>>>>>>> + * @param: Which param to query
>>>>>>>>> + *
>>>>>>>>> + * DMA_BUF_INFO_MAP_INFO:
>>>>>>>>> + * Returns enum dma_buf_map_info, describing the coherency and
>>>>>>>>> + * caching of a CPU mapping of the buffer.
>>>>>>>>> + */
>>>>>>>>> + __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 +161,6 @@ 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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