Re: [PATCH 7/7] swiotlb: don't override the command line in swiotlb_adjust_size

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

On 4/22/21 2:19 AM, Christoph Hellwig wrote:
> When the user specified an explicit swiotlb size on the command line,
> the achitecture code should not override it.
> 
> Fixes: 2cbc2776efe4 ("swiotlb: remove swiotlb_nr_tbl")
> Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

I tested this patchset and I'm not able to get the override via the
command line or via the SEV adjustment function. Looking at the code,
swiotlb_default_size is initialized, so the call to swiotlb_adjust_size()
always returns without setting the new size.

Thanks,
Tom

> ---
>  kernel/dma/swiotlb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 87d06ddf4753f3..aef02a3825b494 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -106,7 +106,9 @@ void swiotlb_set_max_segment(unsigned int val)
>  
>  unsigned long swiotlb_size_or_default(void)
>  {
> -     return swiotlb_default_size;
> +     if (swiotlb_default_size)
> +             return swiotlb_default_size;
> +     return IO_TLB_DEFAULT_SIZE;
>  }
>  
>  void __init swiotlb_adjust_size(unsigned long size)
> @@ -116,6 +118,8 @@ void __init swiotlb_adjust_size(unsigned long size)
>        * architectures such as those supporting memory encryption to
>        * adjust/expand SWIOTLB size for their use.
>        */
> +     if (swiotlb_default_size)
> +             return;
>       swiotlb_default_size = ALIGN(size, IO_TLB_SEGSIZE << IO_TLB_SHIFT);
>       pr_info("SWIOTLB bounce buffer size adjusted to %luMB",
>               swiotlb_default_size >> 20);
>