x86/entry: Add kernel IBRS implementation

commit 2dbb887e upstream.

Implement Kernel IBRS - currently the only known option to mitigate RSB
underflow speculation issues on Skylake hardware.

Note: since IBRS_ENTER requires fuller context established than
UNTRAIN_RET, it must be placed after it. However, since UNTRAIN_RET
itself implies a RET, it must come after IBRS_ENTER. This means
IBRS_ENTER needs to also move UNTRAIN_RET.

Note 2: KERNEL_IBRS is sub-optimal for XenPV.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: conflict at arch/x86/entry/entry_64.S, skip_r11rcx]
[cascardo: conflict at arch/x86/entry/entry_64_compat.S]
[cascardo: conflict fixups, no ANNOTATE_NOENDBR]
[cascardo: entry fixups because of missing UNTRAIN_RET]
[cascardo: conflicts on fsgsbase]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>