RE: [PATCH 1/2] Swiotlb: Add swiotlb_alloc_from_low_pages switch

[Michael Kelley (LINUX)]

From: Tianyu Lan <ltykernel@xxxxxxxxx> Sent: Wednesday, January 26, 2022 8:11 AM
> 
> Hyper-V Isolation VM and AMD SEV VM uses swiotlb bounce buffer to
> share memory with hypervisor. Current swiotlb bounce buffer is only
> allocated from 0 to ARCH_LOW_ADDRESS_LIMIT which is default to
> 0xffffffffUL. Isolation VM and AMD SEV VM needs 1G bounce buffer at most.
> This will fail when there is not enough contiguous memory from 0 to 4G
> address space and devices also may use memory above 4G address space as
> DMA memory. Expose swiotlb_alloc_from_low_pages and platform mey set it
> to false when it's not necessary to limit bounce buffer from 0 to 4G memory.
> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> ---
>  include/linux/swiotlb.h |  1 +
>  kernel/dma/swiotlb.c    | 13 +++++++++++--
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index f6c3638255d5..55c178e8eee0 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -191,5 +191,6 @@ static inline bool is_swiotlb_for_alloc(struct device *dev)
>  #endif /* CONFIG_DMA_RESTRICTED_POOL */
> 
>  extern phys_addr_t swiotlb_unencrypted_base;
> +extern bool swiotlb_alloc_from_low_pages;
> 
>  #endif /* __LINUX_SWIOTLB_H */
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index f1e7ea160b43..159fef80f3db 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -73,6 +73,12 @@ enum swiotlb_force swiotlb_force;
> 
>  struct io_tlb_mem io_tlb_default_mem;
> 
> +/*
> + * Get IO TLB memory from the low pages if swiotlb_alloc_from_low_pages
> + * is set.
> + */
> +bool swiotlb_alloc_from_low_pages = true;
> +
>  phys_addr_t swiotlb_unencrypted_base;
> 
>  /*
> @@ -284,8 +290,11 @@ swiotlb_init(int verbose)
>  	if (swiotlb_force == SWIOTLB_NO_FORCE)
>  		return;
> 
> -	/* Get IO TLB memory from the low pages */
> -	tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> +	if (swiotlb_alloc_from_low_pages)
> +		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
> +	else
> +		tlb = memblock_alloc(bytes, PAGE_SIZE);
> +
>  	if (!tlb)
>  		goto fail;
>  	if (swiotlb_init_with_tbl(tlb, default_nslabs, verbose))
> --
> 2.25.1

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>