[PATCH] x86/sgx: Warn explicitly if X86_FEATURE_SGX_LC is not enabled

[Vladis Dronov <vdronov@redhat.com>]

A kernel requires X86_FEATURE_SGX_LC to be able to create SGX enclaves.
There is quite a number of hardware which has X86_FEATURE_SGX but not
X86_FEATURE_SGX_LC. A kernel running on such a hardware does not create
/dev/sgx* devices silently. Explicitly warn if X86_FEATURE_SGX_LC is not
enabled to properly nofity a user about this condition.

The X86_FEATURE_SGX_LC is a CPU feature that enables LE hash MSRs to be
writable when running native enclaves, i.e. using a custom root key rather
than the Intel proprietary key for enclave signing.

Signed-off-by: Vladis Dronov <vdronov@redhat.com>
---

an out-of-commit-message note:

I've hit this issue myself and have spent some time researching where are
my /dev/sgx* devices on an SGX-enabled hardware, so this is a bit personal.

Links related:
https://github.com/intel/linux-sgx/issues/837
https://patchwork.kernel.org/project/platform-driver-x86/patch/20180827185507.17087-3-jarkko.sakkinen@linux.intel.com/

 arch/x86/kernel/cpu/sgx/driver.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
index 22b65a5f5ec6..df4fbfaa6616 100644
--- a/arch/x86/kernel/cpu/sgx/driver.c
+++ b/arch/x86/kernel/cpu/sgx/driver.c
@@ -150,8 +150,10 @@ int __init sgx_drv_init(void)
 	u64 xfrm_mask;
 	int ret;
 
-	if (!cpu_feature_enabled(X86_FEATURE_SGX_LC))
+	if (!cpu_feature_enabled(X86_FEATURE_SGX_LC)) {
+		pr_err("SGX disabled: SGX launch control is not available.\n");
 		return -ENODEV;
+	}
 
 	cpuid_count(SGX_CPUID, 0, &eax, &ebx, &ecx, &edx);
 
-- 
2.48.1