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

[Joe Jin <joe.jin@oracle.com>]

On 10/25/18 9:10 AM, Boris Ostrovsky wrote:
> On 10/25/18 10:23 AM, Joe Jin wrote:
>> On 10/25/18 4:45 AM, Boris Ostrovsky wrote:
>>> 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?
>> I could not say it's expected behavior, but I think it's reasonable.
> 
> I would expect xen_create_contiguous_region() and
> xen_destroy_contiguous_region() to come in pairs. If a region is
> created, it needs to be destroyed. And vice versa.
> 
> 
>>
>> On allocating, it used __get_free_pages() to allocate memory, if lucky the memory is 
>> DMAable, will not exchange memory with hypervisor, obviously this is not guaranteed.
>>
>> And on freeing it could not be identified if memory from Dom0/guest own memory
>> or hypervisor
> 
> 
> I think it can be. if (!(dev_addr + size - 1 <= dma_mask) ||
> range_straddles_page_boundary()) then it must have come from the
> hypervisor, because that's the check we make in
> xen_swiotlb_alloc_coherent().

This is not true.

dev_addr was came from dma_handle, *dma_handle will be changed  after called
xen_create_contiguous_region():

2590 int xen_create_contiguous_region(phys_addr_t pstart, unsigned int order,        
2591                                  unsigned int address_bits,                     
2592                                  dma_addr_t *dma_handle)                        
2593 {                                                                               
......
2617         success = xen_exchange_memory(1UL << order, 0, in_frames,               
2618                                       1, order, &out_frame,                     
2619                                       address_bits);                            
2620                                                                                 
2621         /* 3. Map the new extent in place of old pages. */                      
2622         if (success)                                                            
2623                 xen_remap_exchanged_ptes(vstart, order, NULL, out_frame);       
2624         else                                                                    
2625                 xen_remap_exchanged_ptes(vstart, order, in_frames, 0);          
2626                                                                                 
2627         spin_unlock_irqrestore(&xen_reservation_lock, flags);                   
2628                                                                                 
2629         *dma_handle = virt_to_machine(vstart).maddr;                            
2630         return success ? 0 : -ENOMEM;                                           
2631 }                                                                               


So means dev_addr check on xen_swiotlb_alloc_coherent() is not same one on
xen_swiotlb_free_coherent().

Thanks,
Joe


> 
> 
> -boris
> 
> 
>> , if don't back memory to hypervisor which will lead hypervisor DMA 
>> memory be used up, then on Dom0/guest, DMA request maybe failed, the worse thing is
>> could not start any new guest.
>>
>> Thanks,
>> Joe
>>
>>> -boris
>>>
>