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

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Date:   Wed, 24 Oct 2018 09:02:46 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     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>,
        Boris Ostrovsky <boris.ostrovsky@...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 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
 2) If it not physically contingous

N.B. The logic in the code is inverted, which leads to all sorts of
confusions."

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