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Date:   Tue, 27 Oct 2020 15:27:27 -0400
From:   Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To:     Elliott Mitchell <ehem+undef@m5p.com>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org, hch@lst.de
Subject: Re: [PATCH] fix swiotlb panic on Xen
Message-ID: <20201027192726.GA13396@char.us.oracle.com>
References: <alpine.DEB.2.21.2010261653320.12247@sstabellini-ThinkPad-T480s>
 <20201027175114.GA32110@mattapan.m5p.com>
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> As the person who first found this and then confirmed this fixes a bug:
> 
> Tested-by: Elliott Mitchell <ehem+xen@m5p.com>

Thank you!!

I changed the title and added the various tags and will put it in
linux-next later this week.

From a1eb2768bf5954d25aa0f0136b38f0aa5d92d984 Mon Sep 17 00:00:00 2001
From: Stefano Stabellini <stefano.stabellini@xilinx.com>
Date: Mon, 26 Oct 2020 17:02:14 -0700
Subject: [PATCH] swiotlb: fix "x86: Don't panic if can not alloc buffer for
 swiotlb"

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.

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

Reported-by: Elliott Mitchell <ehem+xen@m5p.com>
Tested-by: Elliott Mitchell <ehem+xen@m5p.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
CC: stable@vger.kernel.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 kernel/dma/swiotlb.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 465a567678d9..e08cac39c0ba 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -229,6 +229,7 @@ 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();
@@ -260,9 +261,11 @@ 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;
 }
@@ -360,6 +363,7 @@ 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();
 
-- 
2.13.6