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SubjectRe: [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous
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On 10/24/18 10:43 AM, Joe Jin wrote:
> On 10/24/18 6:57 AM, Boris Ostrovsky wrote:
>> 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@oracle.com>
>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>> Reported-by: Boris Ostrovs... ?
>>>> Cc: Christoph Helwig <hch@lst.de>
>>>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
>>>> Cc: John Sobecki <john.sobecki@oracle.com>
>>>> ---
>>>> 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?
> No this is not correct.
>
> When allocate memory, it tried to allocated from Dom0/Guest, then check if physical
> address is DMA memory also contiguous, if no, exchange with Hypervisor, code as below:
>
> 326 phys = *dma_handle;
> 327 dev_addr = xen_phys_to_bus(phys);
> 328 if (((dev_addr + size - 1 <= dma_mask)) &&
> 329 !range_straddles_page_boundary(phys, size))
> 330 *dma_handle = dev_addr;
> 331 else {
> 332 if (xen_create_contiguous_region(phys, order,
> 333 fls64(dma_mask), dma_handle) != 0) {
> 334 xen_free_coherent_pages(hwdev, size, ret, (dma_addr_t)phys, attrs);
> 335 return NULL;
> 336 }
> 337 }
>
>
> On freeing, need to return the memory to Xen, otherwise DMA memory will be used
> up(this is the issue the patch intend to fix), so when memory is DMAable and
> contiguous then call xen_destroy_contiguous_region(), return DMA memory to Xen.

So if you want to allocate 1 byte at address 0 (and dev_addr=phys),
xen_create_contiguous_region() will not be called. And yet you will call
xen_destroy_contiguous_region() in the free path.

Is this the expected behavior?

-boris

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