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

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Message-ID: <83900cf4-690c-9725-d022-d427fdeb4f7d@oracle.com>
Date:   Wed, 24 Oct 2018 09:57:12 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Joe Jin <joe.jin@...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 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?

-boris





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