Re: [RFC V1 4/5] x86: CVMs: Allow allocating all DMA memory from SWIOTLB

[Dave Hansen <dave.hansen@intel.com>]

On 1/11/24 21:52, Vishal Annapurve wrote:
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -112,10 +112,14 @@ void __init mem_encrypt_setup_arch(void)
>  	 * The percentage of guest memory used here for SWIOTLB buffers
>  	 * is more of an approximation of the static adjustment which
>  	 * 64MB for <1G, and ~128M to 256M for 1G-to-4G, i.e., the 6%
> +	 *
> +	 * Extra 2% is added to accommodate the requirement of DMA allocations
> +	 * done using dma_alloc_* APIs.
>  	 */
> -	size = total_mem * 6 / 100;
> -	size = clamp_val(size, IO_TLB_DEFAULT_SIZE, SZ_1G);
> +	size = total_mem * 8 / 100;
> +	size = clamp_val(size, IO_TLB_DEFAULT_SIZE, (SZ_1G + SZ_256M));
>  	swiotlb_adjust_size(size);
> +	swiotlb_adjust_alignment(SZ_2M);

FWIW, this appears superficially to just be fiddling with random
numbers.  The changelog basically says: "did stuff".

What *are* "the requirement of DMA allocations done using dma_alloc_* APIs"?