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

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Date:   Thu, 27 Oct 2022 13:46:32 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, Ashish Kalra <ashish.kalra@....com>,
        Pankaj Gupta <pankaj.gupta@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH kernel] x86/swiotlb/amd: Half the size if allocation
 failed

On Thu, Oct 27, 2022 at 04:26:07PM +1100, 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@....com>

Should this have a
Fixes: e998879d4fb7 ("x86,swiotlb: Adjust SWIOTLB bounce buffer size for SEV guests")
?

Cascardo.

> --
> 
> 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) {
> -- 
> 2.37.3
>