Re: [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous

[Boris Ostrovsky]

On 10/24/18 9:02 AM, Konrad Rzeszutek Wilk wrote:
> On Tue, Oct 23, 2018 at 08:09:04PM -0700, Joe Jin wrote:
>> Commit 4855c92dbb7 "xen-swiotlb: fix the check condition for
>> xen_swiotlb_free_coherent" only fixed memory address check condition
>> on xen_swiotlb_free_coherent(), when memory was not physically
>> contiguous and tried to exchanged with Xen via 
>> xen_destroy_contiguous_region it will lead kernel panic.
> s/it will lead/which lead to/?
>
>> The correct check condition should be memory is in DMA area and
>> physically contiguous.
> "The correct check condition to make Xen hypercall to revert the
> memory back from its 32-bit pool is if it is:
>  1) Above its DMA bit mask (for example 32-bit devices can only address
> up to 4GB, and we may want 4GB+2K), and

Is this "and' or 'or'?

>  2) If it not physically contingous
>
> N.B. The logic in the code is inverted, which leads to all sorts of
> confusions."


I would, in fact, suggest to make the logic the same in both
xen_swiotlb_alloc_coherent() and xen_swiotlb_free_coherent() to avoid
this. This will involve swapping if and else in the former.


>
> Does that sound correct?
>
>> Thank you Boris for pointing it out.
>>
> Fixes: 4855c92dbb7 ("xen-sw..") ?
>
>> Signed-off-by: Joe Jin <joe.jin@xxxxxxxxxx>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Reported-by: Boris Ostrovs... ?
>> Cc: Christoph Helwig <hch@xxxxxx>
>> Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>> Cc: John Sobecki <john.sobecki@xxxxxxxxxx>
>> ---
>>  drivers/xen/swiotlb-xen.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
>> index f5c1af4ce9ab..aed92fa019f9 100644
>> --- a/drivers/xen/swiotlb-xen.c
>> +++ b/drivers/xen/swiotlb-xen.c
>> @@ -357,8 +357,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr,
>>  	/* Convert the size to actually allocated. */
>>  	size = 1UL << (order + XEN_PAGE_SHIFT);
>>  
>> -	if (((dev_addr + size - 1 <= dma_mask)) ||
>> -	    range_straddles_page_boundary(phys, size))
>> +	if ((dev_addr + size - 1 <= dma_mask) &&
>> +	    !range_straddles_page_boundary(phys, size))
>>  		xen_destroy_contiguous_region(phys, order);


I don't think this is right.

if ((dev_addr + size - 1 > dma_mask) || range_straddles_page_boundary(phys, size))

No?

-boris





>>  
>>  	xen_free_coherent_pages(hwdev, size, vaddr, (dma_addr_t)phys, attrs);
>> -- 
>> 2.17.1 (Apple Git-112)
>>