From: Borislav Petkov bp@suse.de
commit 711885906b5c2df90746a51f4cd674f1ab9fbb1d upstream.
This Kconfig option was added initially so that memory encryption is enabled by default on machines which support it.
However, devices which have DMA masks that are less than the bit position of the encryption bit, aka C-bit, require the use of an IOMMU or the use of SWIOTLB.
If the IOMMU is disabled or in passthrough mode, the kernel would switch to SWIOTLB bounce-buffering for those transfers.
In order to avoid that,
2cc13bb4f59f ("iommu: Disable passthrough mode when SME is active")
disables the default IOMMU passthrough mode so that devices for which the default 256K DMA is insufficient, can use the IOMMU instead.
However 2, there are cases where the IOMMU is disabled in the BIOS, etc. (think the usual hardware folk "oops, I dropped the ball there" cases) or a driver doesn't properly use the DMA APIs or a device has a firmware or hardware bug, e.g.:
ea68573d408f ("drm/amdgpu: Fail to load on RAVEN if SME is active")
However 3, in the above GPU use case, there are APIs like Vulkan and some OpenGL/OpenCL extensions which are under the assumption that user-allocated memory can be passed in to the kernel driver and both the GPU and CPU can do coherent and concurrent access to the same memory. That cannot work with SWIOTLB bounce buffers, of course.
So, in order for those devices to function, drop the "default y" for the SME by default active option so that users who want to have SME enabled, will need to either enable it in their config or use "mem_encrypt=on" on the kernel command line.
[ tlendacky: Generalize commit message. ]
Fixes: 7744ccdbc16f ("x86/mm: Add Secure Memory Encryption (SME) support") Reported-by: Paul Menzel pmenzel@molgen.mpg.de Signed-off-by: Borislav Petkov bp@suse.de Acked-by: Alex Deucher alexander.deucher@amd.com Acked-by: Tom Lendacky thomas.lendacky@amd.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/8bbacd0e-4580-3194-19d2-a0ecad7df09c@molgen.mpg.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/Kconfig | 1 - 1 file changed, 1 deletion(-)
--- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -1456,7 +1456,6 @@ config AMD_MEM_ENCRYPT
config AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT bool "Activate AMD Secure Memory Encryption (SME) by default" - default y depends on AMD_MEM_ENCRYPT ---help--- Say yes to have system memory encrypted by default if running on