Re: [PATCH v6 03/20] dma-direct: use DMA_ATTR_CC_SHARED in alloc/free paths

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Alexey Kardashevskiy <aik@amd.com> writes:

> On 4/6/26 18:39, Aneesh Kumar K.V (Arm) wrote:
>> Propagate force_dma_unencrypted() into DMA_ATTR_CC_SHARED in the
>> dma-direct allocation path and use the attribute to drive the related
>> decisions.
>> 
>> This updates dma_direct_alloc(), dma_direct_free(), and
>> dma_direct_alloc_pages() to fold the forced unencrypted case into attrs.
>> 
>> Tested-by: Jiri Pirko <jiri@nvidia.com>
>> Tested-by: Michael Kelley <mhklinux@outlook.com>
>> Tested-by: Mostafa Saleh <smostafa@google.com>
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>   kernel/dma/direct.c | 53 +++++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 44 insertions(+), 9 deletions(-)
>> 
>> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
>> index a741c8a2ee66..90dc5057a0c0 100644
>> --- a/kernel/dma/direct.c
>> +++ b/kernel/dma/direct.c
>> @@ -193,16 +193,31 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>>   		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
>>   {
>>   	bool remap = false, set_uncached = false;
>> -	bool mark_mem_decrypt = true;
>> +	bool mark_mem_decrypt = false;
>>   	struct page *page;
>>   	void *ret;
>>   
>> +	/*
>> +	 * DMA_ATTR_CC_SHARED is not a caller-visible dma_alloc_*()
>> +	 * attribute. The direct allocator uses it internally after it has
>> +	 * decided that the backing pages must be shared/decrypted, so the
>> +	 * rest of the allocation path can consistently select DMA addresses,
>> +	 * choose compatible pools and restore encryption on free.
>
> Why this limit?
>
> Context: I am looking for a memory pool for a few shared pages (to do
> some guest<->host communication), SWIOTLB seems like the right fit but
> swiotlb_alloc() is not exported and
> dma_direct_alloc(DMA_ATTR_CC_SHARED) is not allowed. Thanks,
>

swiotlb is not the right pool to use for that, right?
CCA had a similar requirement for ITS pages and ended up creating a genpool:
b08e2f42e86b ("irqchip/gic-v3-its: Share ITS tables with a non-trusted hypervisor")

-aneesh