Re: [PATCH] fix swiotlb panic on Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 27 Oct 2020, Konrad Rzeszutek Wilk wrote:
> 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.
> 
> Should this have a Fixes: flag?

That would be

Fixes: ac2cbab21f31 ("x86: Don't panic if can not alloc buffer for swiotlb")



> > 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();
> >  
>