i915 "GPU HANG", bisected to a2daa27c0c61 "swiotlb: simplify swiotlb_max_segment"

[Jan Beulich]

On 18.10.2022 10:24, Christoph Hellwig wrote:
> @@ -127,19 +128,22 @@ static inline unsigned int i915_sg_dma_sizes(struct scatterlist *sg)
>  	return page_sizes;
>  }
>  
> -static inline unsigned int i915_sg_segment_size(void)
> +static inline unsigned int i915_sg_segment_size(struct device *dev)
>  {
> -	unsigned int size = swiotlb_max_segment();
> -
> -	if (size == 0)
> -		size = UINT_MAX;
> -
> -	size = rounddown(size, PAGE_SIZE);
> -	/* swiotlb_max_segment_size can return 1 byte when it means one page. */
> -	if (size < PAGE_SIZE)
> -		size = PAGE_SIZE;
> -
> -	return size;
> +	size_t max = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));
> +
> +	/*
> +	 * Xen on x86 can reshuffle pages under us.  The DMA API takes
> +	 * care of that both in dma_alloc_* (by calling into the hypervisor
> +	 * to make the pages contigous) and in dma_map_* (by bounce buffering).
> +	 * But i915 abuses ignores the coherency aspects of the DMA API and
> +	 * thus can't cope with bounce buffering actually happening, so add
> +	 * a hack here to force small allocations and mapping when running on
> +	 * Xen.  (good luck with TDX, btw --hch)
> +	 */
> +	if (IS_ENABLED(CONFIG_X86) && xen_domain())
> +		max = PAGE_SIZE;
> +	return round_down(max, PAGE_SIZE);
>  }

Shouldn't this then be xen_pv_domain() that you use here, and - if you
really want IS_ENABLED() in addition - CONFIG_XEN_PV?

Jan