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Date:   Tue, 27 Oct 2020 10:51:14 -0700
From:   Elliott Mitchell <ehem+undef@m5p.com>
To:     Stefano Stabellini <sstabellini@kernel.org>
Cc:     iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org, konrad.wilk@oracle.com, hch@lst.de
Subject: Re: [PATCH] fix swiotlb panic on Xen
Message-ID: <20201027175114.GA32110@mattapan.m5p.com>
References: <alpine.DEB.2.21.2010261653320.12247@sstabellini-ThinkPad-T480s>
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On Mon, Oct 26, 2020 at 05:02:14PM -0700, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> kernel/dma/swiotlb.c:swiotlb_init gets called first and tries to
> allocate a buffer for the swiotlb. It does so by calling
> 
>   memblock_alloc_low(PAGE_ALIGN(bytes), PAGE_SIZE);
> 
> If the allocation must fail, no_iotlb_memory is set.
> 
> 
> Later during initialization swiotlb-xen comes in
> (drivers/xen/swiotlb-xen.c:xen_swiotlb_init) and given that io_tlb_start
> is != 0, it thinks the memory is ready to use when actually it is not.
> 
> When the swiotlb is actually needed, swiotlb_tbl_map_single gets called
> and since no_iotlb_memory is set the kernel panics.
> 
> Instead, if swiotlb-xen.c:xen_swiotlb_init knew the swiotlb hadn't been
> initialized, it would do the initialization itself, which might still
> succeed.
> 
> 
> Fix the panic by setting io_tlb_start to 0 on swiotlb initialization
> failure, and also by setting no_iotlb_memory to false on swiotlb
> initialization success.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c19379fabd20..9924214df60a 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -231,6 +231,7 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
>  		io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
>  	}
>  	io_tlb_index = 0;
> +	no_iotlb_memory = false;
>  
>  	if (verbose)
>  		swiotlb_print_info();
> @@ -262,9 +263,11 @@ swiotlb_init(int verbose)
>  	if (vstart && !swiotlb_init_with_tbl(vstart, io_tlb_nslabs, verbose))
>  		return;
>  
> -	if (io_tlb_start)
> +	if (io_tlb_start) {
>  		memblock_free_early(io_tlb_start,
>  				    PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));
> +		io_tlb_start = 0;
> +	}
>  	pr_warn("Cannot allocate buffer");
>  	no_iotlb_memory = true;
>  }
> @@ -362,6 +365,7 @@ swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs)
>  		io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
>  	}
>  	io_tlb_index = 0;
> +	no_iotlb_memory = false;
>  
>  	swiotlb_print_info();
>  

As the person who first found this and then confirmed this fixes a bug:

Tested-by: Elliott Mitchell <ehem+xen@m5p.com>


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