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

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Message-ID: <c3a52494-7ced-42ec-8839-11bd18a7c3d9@suse.com>
Date: Mon, 16 Sep 2024 09:01:35 +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 08:59, 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.

Jan