x86/entry: Adjust guest paths to be shadow stack compatible

x86/entry: Adjust guest paths to be shadow stack compatible

The SYSCALL/SYSENTER/SYSRET paths need to use {SET,CLR}SSBSY.  The IRET to
guest paths must not.  In the SYSRET path, re-position the mov which loads rip
into %rcx so we can use %rcx for CLRSSBSY, rather than spilling another
register to the stack.

While we can in principle detect shadow stack corruption and a failure to
clear the supervisor token busy bit in the SYSRET path (by inspecting the
carry flag following CLRSSBSY), we cannot detect similar problems for the IRET
path (IRET is specified not to fault in this case).

We will double fault at some point later, when next trying to enter Xen, due
to an already-set supervisor shadow stack busy bit.  As SYSRET is a uncommon
path anyway, avoid the added complexity for no appreciable gain.

The IST switch onto the primary stack is not great as we have an instruction
boundary with no shadow stack.  This is the least bad option available.

These paths are not used before shadow stacks are properly established, so can
use alternatives to avoid extra runtime CET detection logic.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>