Re: [PATCH v6 7/9] dma-buf: system_heap: Enforce shared-granule alignment for cc-shared buffers

[Christian König]

On 9/18/26 17:36, Jason Gunthorpe wrote:
> On Fri, Sep 18, 2026 at 05:23:32PM +0200, Christian König wrote:
>> On 9/18/26 17:16, Catalin Marinas wrote:
>>> First, none of the DMA-BUF maintainers have been cc'ed. It might be fine
>>> for an RFC but this series got to version 6. We need their feedback.
>>
>> Yeah, thanks for doing this.
>>> On Fri, Sep 04, 2026 at 04:04:50PM +0530, Aneesh Kumar K.V (Arm) wrote:
>>>> The system heap can allocate buffers that are decrypted and shared with the
>>>> host. For confidential-computing guests, those shared buffers must cover
>>>> whole shared-buffer granule; otherwise a userspace mmap of the dma-buf may
>>>> expose only part of a host-managed granule and allow unintended access to
>>>> adjacent private memory.
>>>>
>>>> Require cc-shared system-heap allocations to have a size aligned to
>>>> mem_cc_shared_granule_size(), and allocate pages at least as large as the
>>>> required granule. Keep the allocation bounded by the existing heap orders,
>>>> but fall back to an exact minimum-order allocation when the required
>>>> granule is not one of the preferred heap orders.
>>
>> Uff, I don't think we can do that.
>>
>> That is massively platform specific behavior in a non platform
>> specific code.
> 
> What we really need is an allocator API that does all this for the caller.
> 
> It has been pointed out a few times, Aneesh maybe you need to try to
> tackle that?
> 
> dmabuf heap just wants
> 
> 'allocate me an array of shared folios totaling XX bytes'

Yeah, I know at least two other use cases for that as well.

> Arch code can figure out how to do it. If some ARM configs only give
> order 4 folios or whatever then dmabuf heap doesn't care.

The fundamental problem is that DMA allocations are highly architecture and device specific while Linux memory allocation APIs are generic.

Christian.

> 
> And solve the double/triple zeroing problem.
> 
> Jason