swiotlb: Enable it under x86 PAE

Most distributions end up enabling SWIOTLB already with 32-bit
kernels due to the combination of CONFIG_HYPERVISOR_GUEST|CONFIG_XEN=y
as those end up requiring the SWIOTLB.

However for those that are not interested in virtualization and
run in 32-bit they will discover that: "32-bit PAE 4.2.0 kernel
(no IOMMU code) would hang when writing to my USB disk. The kernel
spews million(-ish messages per sec) to syslog, effectively
"hanging" userspace with my kernel.

Oct  2 14:33:06 voodoochild kernel: [  223.287447] nommu_map_sg:
overflow 25dcac000+1024 of device mask ffffffff
Oct  2 14:33:06 voodoochild kernel: [  223.287448] nommu_map_sg:
overflow 25dcac000+1024 of device mask ffffffff
Oct  2 14:33:06 voodoochild kernel: [  223.287449] nommu_map_sg:
overflow 25dcac000+1024 of device mask ffffffff
... etc ..."

Enabling it makes the problem go away.

N.B. With a6dfa128ce5c414ab46b1d690f7a1b8decb8526d
"config: Enable NEED_DMA_MAP_STATE by default when SWIOTLB is selected"
we also have the important part of the SG macros enabled to make this
work properly - in case anybody wants to backport this patch.

Reported-and-Tested-by: Christian Melki <christian.melki@t2data.com>
Signed-off-by: Christian Melki <christian.melki@t2data.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 328c8352480c..96d058a87100 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1308,6 +1308,7 @@ config HIGHMEM
 config X86_PAE
         bool "PAE (Physical Address Extension) Support"
         depends on X86_32 && !HIGHMEM4G
+        select SWIOTLB
         ---help---
           PAE is required for NX support, and furthermore enables
           larger swapspace support for non-overcommit purposes. It