linux-kernel - Re: [PATCH v2 1/2] xen/swiotlb: add alignment check for dma buffers

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Message-ID: <c97450a0-6ddd-49c5-a8e4-0cd2cf9d1635@suse.com>
Date: Mon, 16 Sep 2024 08:59:44 +0200
From: Jan Beulich <jbeulich@...e.com>
To: Juergen Gross <jgross@...e.com>
Cc: Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 iommu@...ts.linux.dev
Subject: Re: [PATCH v2 1/2] xen/swiotlb: add alignment check for dma buffers

On 16.09.2024 08:56, Juergen Gross wrote:
> On 16.09.24 08:50, Jan Beulich wrote:
>> On 16.09.2024 08:47, 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);
>>> +	phys_addr_t algn = 1ULL << (get_order(size) + PAGE_SHIFT);
>>>   
>>>   	next_bfn = pfn_to_bfn(xen_pfn);
>>>   
>>> +	/* If buffer is physically aligned, ensure DMA alignment. */
>>> +	if (IS_ALIGNED(p, algn) &&
>>> +	    !IS_ALIGNED(next_bfn << XEN_PAGE_SHIFT, algn))
>>
>> And this shift is not at risk of losing bits on Arm LPAE?
> 
> For alignment check this just doesn't matter (assuming XEN_PAGE_SIZE is
> smaller than 4G).

Oh, yes - of course. A (hypothetical?) strict no-overflow checking
mode of the kernel may take issue though, so maybe better to right-
shift "algn"?

Jan