Re: [PATCH kernel] x86/swiotlb/amd: Half the size if allocation failed

[Tom Lendacky]

On 10/27/22 00:26, Alexey Kardashevskiy wrote:
> At the moment the AMD encrypted platform reserves 6% of RAM for SWIOTLB
> or 1GB, whichever is less. However it is possible that there is no block
> big enough in the low memory which make SWIOTLB allocation fail and
> the kernel continues without DMA. In such case a VM hangs on DMA.
>
> This divides the size in half and tries again reusing the existing
> remapping logic.
>
> This updates default_nslabs on successful allocation which looks like
> an oversight as not doing so should have broken callers of
> swiotlb_size_or_default().
>
> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> --
>
> I hit the problem with
> QEMU's "-m 16819M" where SWIOTLB was adjusted to
> 0x7e200 == 1,058,013,184 (slightly less than 1GB) while
> 0x7e180 still worked.
>
> With guest errors enabled, there are many unassigned accesses from
> virtio.
>
> ---
>   kernel/dma/swiotlb.c | 20 +++++++++++++-------
>   1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 339a990554e7..d28c294320fd 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -338,21 +338,27 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
>   	else
>   		tlb = memblock_alloc_low(bytes, PAGE_SIZE);
>   	if (!tlb) {
> -		pr_warn("%s: failed to allocate tlb structure\n", __func__);
> -		return;
> -	}
> -
> -	if (remap && remap(tlb, nslabs) < 0) {
> +		pr_warn("%s: Failed to allocate %zu bytes tlb structure\n",
> +			__func__, bytes);
> +	} else if (remap && remap(tlb, nslabs) < 0) {
>   		memblock_free(tlb, PAGE_ALIGN(bytes));
> +		tlb = NULL;
> +		pr_warn("%s: Failed to remap %zu bytes\n", __func__, bytes);
> +	}
>   
> +	if (!tlb) {
>   		nslabs = ALIGN(nslabs >> 1, IO_TLB_SEGSIZE);
>   		if (nslabs >= IO_TLB_MIN_SLABS)
>   			goto retry;
> -
> -		pr_warn("%s: Failed to remap %zu bytes\n", __func__, bytes);
>   		return;
>   	}
>   
> +	if (default_nslabs != nslabs) {
> +		pr_info("SWIOTLB bounce buffer size adjusted %lu -> %lu slabs",
> +			default_nslabs, nslabs);
> +		default_nslabs = nslabs;
> +	}
> +
>   	alloc_size = PAGE_ALIGN(array_size(sizeof(*mem->slots), nslabs));
>   	mem->slots = memblock_alloc(alloc_size, PAGE_SIZE);
>   	if (!mem->slots) {