Re: [PATCH V1] accel/amdxdna: Support read-only user-pointer BO mappings

[Lizhi Hou <lizhi.hou@amd.com>]

On 3/31/26 09:13, Mario Limonciello wrote:
>
>
> On 3/26/26 11:27, Lizhi Hou wrote:
>> From: Max Zhen <max.zhen@amd.com>
>>
>> Update the amdxdna user-pointer (ubuf) BO path to support creating 
>> buffer
>> objects from read-only user mappings.
>>
>> Detect read-only VMAs by checking VMA permissions across all user 
>> virtual
>> address ranges associated with the BO. When all entries are 
>> read-only, pin
>> user pages without FOLL_WRITE and export the resulting dmabuf as 
>> read-only
>> (O_RDONLY).
>>
>> This allows userptr BOs backed by read-only mappings to be safely 
>> imported
>> and used without requiring write access, which was previously 
>> rejected due
>> to unconditional FOLL_WRITE usage.
>>
>> Signed-off-by: Max Zhen <max.zhen@amd.com>
>> Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
>> ---
>>   drivers/accel/amdxdna/amdxdna_ubuf.c | 30 ++++++++++++++++++++++++++--
>>   1 file changed, 28 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/accel/amdxdna/amdxdna_ubuf.c 
>> b/drivers/accel/amdxdna/amdxdna_ubuf.c
>> index 4c0647057759..1a0e2a274170 100644
>> --- a/drivers/accel/amdxdna/amdxdna_ubuf.c
>> +++ b/drivers/accel/amdxdna/amdxdna_ubuf.c
>> @@ -125,6 +125,27 @@ static const struct dma_buf_ops 
>> amdxdna_ubuf_dmabuf_ops = {
>>       .vunmap = amdxdna_ubuf_vunmap,
>>   };
>>   +static int readonly_va_entry(struct amdxdna_drm_va_entry *va_ent)
>> +{
>> +    struct mm_struct *mm = current->mm;
>> +    struct vm_area_struct *vma;
>> +    int ret;
>> +
>> +    mmap_read_lock(mm);
>> +
>> +    vma = find_vma(mm, va_ent->vaddr);
>> +    if (!vma ||
>> +        vma->vm_start > va_ent->vaddr ||
>> +        vma->vm_end < va_ent->vaddr ||
>> +        vma->vm_end - va_ent->vaddr < va_ent->len)
>> +        ret = -ENOENT;
>
> The check on line "vma->vm_end < va_ent->vaddr" appears to be 
> unreachable.
> find_vma() is documented to return the first VMA where vma->vm_end > 
> addr, so
> if vma is non-NULL, this condition can never be true.
Sure. I will remove it.
>
>> +    else
>> +        ret = vma->vm_flags & VM_WRITE ? 0 : 1;
>> +
>> +    mmap_read_unlock(mm);
>> +    return ret;
>> +}
>> +
>>   struct dma_buf *amdxdna_get_ubuf(struct drm_device *dev,
>>                    u32 num_entries, void __user *va_entries)
>>   {
>> @@ -134,6 +155,7 @@ struct dma_buf *amdxdna_get_ubuf(struct 
>> drm_device *dev,
>>       struct amdxdna_ubuf_priv *ubuf;
>>       u32 npages, start = 0;
>>       struct dma_buf *dbuf;
>> +    bool readonly = true;
>>       int i, ret;
>>       DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
>>   @@ -172,6 +194,10 @@ struct dma_buf *amdxdna_get_ubuf(struct 
>> drm_device *dev,
>>               ret = -EINVAL;
>>               goto free_ent;
>>           }
>> +
>> +        /* Pin pages as writable as long as not all entries are 
>> read-only. */
>> +        if (readonly && readonly_va_entry(&va_ent[i]) != 1)
>> +            readonly = false;
>>       }
>
> The check "!= 1" treats errors the same as writable VMAs. 
> readonly_va_entry()
> returns -ENOENT for errors, 0 for writable, and 1 for read-only.
>
> Extra error handling maybe needed?

The idea is to specifically handle the readonly_va_entry() == 1 case. 
For -ENOENT and 0, it fallbacks to writeable and the error handling will 
be done by checking pin_user_pages_fast() return value.


Lizhi

>
>>         ubuf->nr_pages = exp_info.size >> PAGE_SHIFT;
>> @@ -194,7 +220,7 @@ struct dma_buf *amdxdna_get_ubuf(struct 
>> drm_device *dev,
>>           npages = va_ent[i].len >> PAGE_SHIFT;
>>             ret = pin_user_pages_fast(va_ent[i].vaddr, npages,
>> -                      FOLL_WRITE | FOLL_LONGTERM,
>> +                      (readonly ? 0 : FOLL_WRITE) | FOLL_LONGTERM,
>>                         &ubuf->pages[start]);
>>           if (ret >= 0) {
>>               start += ret;
>> @@ -211,7 +237,7 @@ struct dma_buf *amdxdna_get_ubuf(struct 
>> drm_device *dev,
>>         exp_info.ops = &amdxdna_ubuf_dmabuf_ops;
>>       exp_info.priv = ubuf;
>> -    exp_info.flags = O_RDWR | O_CLOEXEC;
>> +    exp_info.flags = (readonly ? O_RDONLY : O_RDWR) | O_CLOEXEC;
>>         dbuf = dma_buf_export(&exp_info);
>>       if (IS_ERR(dbuf)) {
>