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

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Date:   Thu, 25 Oct 2018 07:45:54 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Joe Jin <joe.jin@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     "DONGLI.ZHANG" <dongli.zhang@...cle.com>, konrad@...nel.org,
        Christoph Helwig <hch@....de>,
        John Sobecki <john.sobecki@...cle.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org\"" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen-swiotlb: exchange memory with Xen only when pages are
 contiguous

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@...cle.com>
>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Reported-by: Boris Ostrovs... ?
>>>> Cc: Christoph Helwig <hch@....de>
>>>> Cc: Dongli Zhang <dongli.zhang@...cle.com>
>>>> Cc: John Sobecki <john.sobecki@...cle.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