AMX TILEDATA is a very large XSAVE feature.  It could have caused
nasty XSAVE buffer space waste in two places:

 * Signal stacks
 * Kernel task_struct->fpu buffers

To avoid this waste, neither of these buffers have AMX state by
default.  The non-default features are called "dynamic" features.

There is an arch_prctl(ARCH_REQ_XCOMP_PERM) which allows a task
to declare that it wants to use AMX or other "dynamic" XSAVE
features.  This arch_prctl() ensures that sufficient sigaltstack
space is available before it will succeed.  It also expands the
task_struct buffer.

Functions of this test:
 * Test arch_prctl(ARCH_REQ_XCOMP_PERM).  Ensure that it checks for
   proper sigaltstack sizing and that the sizing is enforced for
   future sigaltstack calls.
 * Ensure that ARCH_REQ_XCOMP_PERM is inherited across fork()
 * Ensure that TILEDATA use before the prctl() is fatal
 * Ensure that TILEDATA is cleared across fork()

Note: Generally, compiler support is needed to do something with
AMX.  Instead, directly load AMX state from userspace with a
plain XSAVE.  Do not depend on the compiler.

 [ dhansen: bunches of cleanups ]
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211026122524.7BEDAA95@davehans-spike.ostc.intel.com