ACK: [PATCH] UBUNTU: SAUCE: swiotlb: Max mapping size takes min align mask into account

[Thadeu Lima de Souza Cascardo]

On Thu, May 12, 2022 at 06:07:06AM -0600, Tim Gardner wrote:
> From: Tianyu Lan <Tianyu.Lan at microsoft.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1973169
> 
> swiotlb_find_slots() skips slots according to io tlb aligned mask
> calculated from min aligned mask and original physical address
> offset. This affects max mapping size. The mapping size can't
> achieve the IO_TLB_SEGSIZE * IO_TLB_SIZE when original offset is
> non-zero. This will cause system boot up failure in Hyper-V
> Isolation VM where swiotlb force is enabled. Scsi layer use return
> value of dma_max_mapping_size() to set max segment size and it
> finally calls swiotlb_max_mapping_size(). Hyper-V storage driver
> sets min align mask to 4k - 1. Scsi layer may pass 256k length of
> request buffer with 0~4k offset and Hyper-V storage driver can't
> get swiotlb bounce buffer via DMA API. Swiotlb_find_slots() can't
> find 256k length bounce buffer with offset. Make swiotlb_max_mapping
> _size() take min align mask into account.
> 
> (patch merged from https://lore.kernel.org/lkml/20220511060212.GA32192@lst.de/T/)
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan at microsoft.com>
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  kernel/dma/swiotlb.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index ce241d72ad03..570af75332f5 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -766,7 +766,18 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t paddr, size_t size,
>  
>  size_t swiotlb_max_mapping_size(struct device *dev)
>  {
> -	return ((size_t)IO_TLB_SIZE) * IO_TLB_SEGSIZE;
> +	int min_align_mask = dma_get_min_align_mask(dev);
> +	int min_align = 0;
> +
> +	/*
> +	 * swiotlb_find_slots() skips slots according to
> +	 * min align mask. This affects max mapping size.
> +	 * Take it into acount here.
> +	 */
> +	if (min_align_mask)
> +		min_align = roundup(min_align_mask, IO_TLB_SIZE);
> +
> +	return ((size_t)IO_TLB_SIZE) * IO_TLB_SEGSIZE - min_align;
>  }
>  

I don't like this much, not sure how dma_get_min_align_mask is bound. But given
this is restricted to linux-azure and the mailing list discussion is leaning
torwards accepting such a fix:

Acked-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>

>  bool is_swiotlb_active(struct device *dev)
> -- 
> 2.36.0
> 
> 
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