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    x86/Kconfig: Do not enable AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT automatically · 71188590
    Borislav Petkov 提交于
    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,
    
      2cc13bb4 ("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.:
    
      ea68573d ("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: 7744ccdb ("x86/mm: Add Secure Memory Encryption (SME) support")
    Reported-by: NPaul Menzel <pmenzel@molgen.mpg.de>
    Signed-off-by: NBorislav Petkov <bp@suse.de>
    Acked-by: NAlex Deucher <alexander.deucher@amd.com>
    Acked-by: NTom Lendacky <thomas.lendacky@amd.com>
    Cc: <stable@vger.kernel.org>
    Link: https://lkml.kernel.org/r/8bbacd0e-4580-3194-19d2-a0ecad7df09c@molgen.mpg.de
    71188590
Kconfig 91.9 KB