Re: [PATCH] fix swiotlb panic on Xen

[Elliott Mitchell <ehem+undef@xxxxxxx>]

On Mon, Oct 26, 2020 at 05:02:14PM -0700, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> 
> 
> 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@xxxxxxx>


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