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

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Message-ID: <8ef4ee94-5220-44a3-8095-30c3818ab46e@suse.com>
Date: Mon, 16 Sep 2024 09:03:26 +0200
From: Jürgen Groß <jgross@...e.com>
To: Jan Beulich <jbeulich@...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 16.09.24 09:01, Jan Beulich wrote:
> On 16.09.2024 08:59, Juergen Gross wrote:
>> On 16.09.24 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).
>>
>> Wait, that was nonsense.
> 
> I think it was quite reasonable, as long as also algn (and hence size)
> isn't absurdly large.

Better safe than sorry.


Juergen


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