Re: [PATCH] xen/swiotlb: add alignment check for dma buffers

[Jan Beulich <jbeulich@suse.com>]

On 14.09.2024 02:38, Stefano Stabellini wrote:
> On Fri, 13 Sep 2024, Jan Beulich wrote:
>> On 13.09.2024 16:56, Juergen Gross wrote:
>>> --- a/drivers/xen/swiotlb-xen.c
>>> +++ b/drivers/xen/swiotlb-xen.c
>>> @@ -78,9 +78,15 @@ static inline int range_straddles_page_boundary(phys_addr_t p, size_t size)
>>>  {
>>>  	unsigned long next_bfn, xen_pfn = XEN_PFN_DOWN(p);
>>>  	unsigned int i, nr_pages = XEN_PFN_UP(xen_offset_in_page(p) + size);
>>> +	unsigned int order = get_order(size);
>>>  
>>>  	next_bfn = pfn_to_bfn(xen_pfn);
>>>  
>>> +	/* If buffer is physically aligned, ensure DMA alignment. */
>>> +	if (IS_ALIGNED(p, 1UL << (order + PAGE_SHIFT)) &&
>>
>> Why this check? xen_swiotlb_alloc_coherent() guarantees it, while
>> xen_swiotlb_free_coherent() only checks properties of the original
>> allocation. And for xen_swiotlb_map_page() this looks actively
>> wrong to me, in case that function was called with offset non-zero.
> 
> I understand xen_swiotlb_alloc_coherent and xen_swiotlb_free_coherent
> not needing the check, but I think we might need the check for
> xen_swiotlb_map_page. At that point, I would keep the check for all
> callers.

Whereas I would be inclined to suggest to put it in the one place it's
needed, not the least to avoid the abuse of the function (going just
from its name).

> Unless there is another way to detect whether the mapping needs
> alignment specifically for map_page?
> 
> For the offset, in theory if the device needs alignment, the offset
> should be zero? If the offset is not zero, then there should be no
> alignment requirement. The way Juergen wrote the check, we would take
> the fast path if offset != zero, which makes sense to me.

Hmm, right.

Jan