Re: [PATCH 2/2] swiotlb: Return consistent SWIOTLB segments/nr_tbl

Christoph Hellwig
2019-06-14T09:48:04Z

On Tue, Jun 11, 2019 at 10:58:25AM -0700, Florian Fainelli wrote:
> With a specifically contrived memory layout where there is no physical
> memory available to the kernel below the 4GB boundary, we will fail to
> perform the initial swiotlb_init() call and set no_iotlb_memory to true.
>
> There are drivers out there that call into swiotlb_nr_tbl() to determine
> whether they can use the SWIOTLB. With the right DMA_BIT_MASK() value
> for these drivers (say 64-bit), they won't ever need to hit
> swiotlb_tbl_map_single() so this can go unoticed and we would be
> possibly lying about those drivers.
>
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> ---
> kernel/dma/swiotlb.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b2b5c5df273c..e906ef2e6315 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -129,15 +129,17 @@ setup_io_tlb_npages(char *str)
> }
> early_param("swiotlb", setup_io_tlb_npages);
>
> +static bool no_iotlb_memory;
> +
> unsigned long swiotlb_nr_tbl(void)
> {
> - return io_tlb_nslabs;
> + return unlikely(no_iotlb_memory) ? 0 : io_tlb_nslabs;
> }
> EXPORT_SYMBOL_GPL(swiotlb_nr_tbl);
>
> unsigned int swiotlb_max_segment(void)
> {
> - return max_segment;
> + return unlikely(no_iotlb_memory) ? 0 : max_segment;

I wouldn't bother with the unlikely here as anythign querying
swiotlb details should pretty much be a slow path already.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>