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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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@xxxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Reported-by: Boris Ostrovs... ?
> Cc: Christoph Helwig <hch@xxxxxx>
> Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> Cc: John Sobecki <john.sobecki@xxxxxxxxxx>
> ---
>  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)
> 

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