Re: [RFC PATCH] arm64: swiotlb: dma: its: Ensure shared buffers are properly aligned

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

Catalin Marinas <catalin.marinas@arm.com> writes:

> Hi Aneesh,
>
> On Fri, Sep 05, 2025 at 11:24:41AM +0530, Aneesh Kumar K.V (Arm) wrote:
>> When running with private memory guests, the guest kernel must allocate
>> memory with specific constraints when sharing it with the hypervisor.
>> 
>> These shared memory buffers are also accessed by the host kernel, which
>> means they must be aligned to the host kernel's page size.
>
> So this is the case where the guest page size is smaller than the host
> one. Just trying to understand what would go wrong if we don't do
> anything here. Let's say the guest uses 4K pages and the host a 64K
> pages. Within a 64K range, only a 4K is shared/decrypted. If the host
> does not explicitly access the other 60K around the shared 4K, can
> anything still go wrong? Is the hardware ok with speculative loads from
> non-shared ranges?
>

With features like guest_memfd, the goal is to explicitly prevent the
host from mapping private memory, rather than relying on the host to
avoid accessing those regions.

As per Arm ARM:
RVJLXG: Accesses are checked against the GPC configuration for the
physical granule being accessed, regardless of the stage 1 and stage 2
translation configuration.

For example, if GPCCR_EL3.PGS is configured to a smaller granule size
than the configured stage 1 and stage 2 translation granule size,
accesses are checked at the GPCCR_EL3.PGS granule size.

>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index ea84a61ed508..389070e9ee65 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -337,12 +337,14 @@ void __init bootmem_init(void)
>>  
>>  void __init arch_mm_preinit(void)
>>  {
>> +	unsigned int swiotlb_align = PAGE_SIZE;
>>  	unsigned int flags = SWIOTLB_VERBOSE;
>>  	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>>  
>>  	if (is_realm_world()) {
>>  		swiotlb = true;
>>  		flags |= SWIOTLB_FORCE;
>> +		swiotlb_align = arch_shared_mem_alignment();
>>  	}
>>  
>>  	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
>> @@ -356,7 +358,7 @@ void __init arch_mm_preinit(void)
>>  		swiotlb = true;
>>  	}
>>  
>> -	swiotlb_init(swiotlb, flags);
>> +	swiotlb_init(swiotlb, swiotlb_align, flags);
>>  	swiotlb_update_mem_attributes();
>
> I think there's too much change just to pass down an alignment. We have
> IO_TLB_MIN_SLABS, we could add an IO_TLB_MIN_ALIGN that's PAGE_SIZE by
> default but can be overridden to something dynamic for arm64.
>
>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>> index 467cb78435a9..e2142bbca13b 100644
>> --- a/drivers/irqchip/irq-gic-v3-its.c
>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>> @@ -213,16 +213,20 @@ static gfp_t gfp_flags_quirk;
>>  static struct page *its_alloc_pages_node(int node, gfp_t gfp,
>>  					 unsigned int order)
>>  {
>> +	long new_order;
>>  	struct page *page;
>>  	int ret = 0;
>>  
>> -	page = alloc_pages_node(node, gfp | gfp_flags_quirk, order);
>> +	/* align things to hypervisor page size */
>> +	new_order = get_order(ALIGN((PAGE_SIZE << order), arch_shared_mem_alignment()));
>> +
>> +	page = alloc_pages_node(node, gfp | gfp_flags_quirk, new_order);
>>  
>>  	if (!page)
>>  		return NULL;
>>  
>>  	ret = set_memory_decrypted((unsigned long)page_address(page),
>> -				   1 << order);
>> +				   1 << new_order);
>
> At some point this could move to the DMA API.
>
>> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
>> index 24c359d9c879..5db5baad5efa 100644
>> --- a/kernel/dma/direct.c
>> +++ b/kernel/dma/direct.c
>> @@ -255,6 +255,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>>  	    dma_direct_use_pool(dev, gfp))
>>  		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>>  
>> +	if (force_dma_unencrypted(dev))
>> +		/*  align to hypervisor size */
>> +		size = ALIGN(size, arch_shared_mem_alignment());
>>  	/* we always manually zero the memory once we are done */
>>  	page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO, true);
>
> You align the size but does __dma_direct_alloc_pages() guarantee a
> natural alignment? Digging through cma_alloc_aligned(), it guarantees
> the minimum of size and CONFIG_CMA_ALIGNMENT. The latter can be
> configured to a page order of 2.
>

I will check this.

>
>> @@ -382,7 +384,7 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
>>  
>>  	nslabs = default_nslabs;
>>  	nareas = limit_nareas(default_nareas, nslabs);
>> -	while ((tlb = swiotlb_memblock_alloc(nslabs, flags, remap)) == NULL) {
>> +	while ((tlb = swiotlb_memblock_alloc(nslabs, alignment, flags, remap)) == NULL) {
>>  		if (nslabs <= IO_TLB_MIN_SLABS)
>>  			return;
>>  		nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
>> @@ -417,9 +419,9 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
>>  		swiotlb_print_info();
>>  }
>
> We also have the dynamic swiotlb allocations via swiotlb_dyn_alloc() ->
> swiotlb_alloc_pool() -> swiotlb_alloc_tlb(). I don't see any alignment
> guarantees.
>
> TBH, I'm not too keen on this patch. It feels like a problem to be
> solved by the host - don't advertise smaller page sizes to guest or cope
> with sub-page memory sharing. Currently the streaming DMA is handled via
> bouncing but we may, at some point, just allow set_memory_decrypted() on
> the DMA-mapped page. The above requirements will not allow this.
>
> I cannot come up with a better solution though. I hope the hardware can
> cope with sub-page shared/non-shared ranges.
>

-aneesh