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-Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
-which is found on Intel's Skylake "Scalable Processor" Server CPUs.
-It will be avalable in future non-server parts.
-
-For anyone wishing to test or use this feature, it is available in
-Amazon's EC2 C5 instances and is known to work there using an Ubuntu
-17.04 image.
-
-Memory Protection Keys provides a mechanism for enforcing page-based
-protections, but without requiring modification of the page tables
-when an application changes protection domains.  It works by
-dedicating 4 previously ignored bits in each page table entry to a
-"protection key", giving 16 possible keys.
-
-There is also a new user-accessible register (PKRU) with two separate
-bits (Access Disable and Write Disable) for each key.  Being a CPU
-register, PKRU is inherently thread-local, potentially giving each
-thread a different set of protections from every other thread.
-
-There are two new instructions (RDPKRU/WRPKRU) for reading and writing
-to the new register.  The feature is only available in 64-bit mode,
-even though there is theoretically space in the PAE PTEs.  These
-permissions are enforced on data access only and have no effect on
-instruction fetches.
-
-=========================== Syscalls ===========================
-
-There are 3 system calls which directly interact with pkeys:
-
-	int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
-	int pkey_free(int pkey);
-	int pkey_mprotect(unsigned long start, size_t len,
-			  unsigned long prot, int pkey);
-
-Before a pkey can be used, it must first be allocated with
-pkey_alloc().  An application calls the WRPKRU instruction
-directly in order to change access permissions to memory covered
-with a key.  In this example WRPKRU is wrapped by a C function
-called pkey_set().
-
-	int real_prot = PROT_READ|PROT_WRITE;
-	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);
-	ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
-	ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);
-	... application runs here
-
-Now, if the application needs to update the data at 'ptr', it can
-gain access, do the update, then remove its write access:
-
-	pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE
-	*ptr = foo; // assign something
-	pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again
-
-Now when it frees the memory, it will also free the pkey since it
-is no longer in use:
-
-	munmap(ptr, PAGE_SIZE);
-	pkey_free(pkey);
-
-(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
- An example implementation can be found in
- tools/testing/selftests/x86/protection_keys.c)
-
-=========================== Behavior ===========================
-
-The kernel attempts to make protection keys consistent with the
-behavior of a plain mprotect().  For instance if you do this:
-
-	mprotect(ptr, size, PROT_NONE);
-	something(ptr);
-
-you can expect the same effects with protection keys when doing this:
-
-	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
-	pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
-	something(ptr);
-
-That should be true whether something() is a direct access to 'ptr'
-like:
-
-	*ptr = foo;
-
-or when the kernel does the access on the application's behalf like
-with a read():
-
-	read(fd, ptr, 1);
-
-The kernel will send a SIGSEGV in both cases, but si_code will be set
-to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
-the plain mprotect() permissions are violated.