Despite RISC-V having a direct 'fence.i' instruction available to
userspace (which we can't trap!), that's not actually viable when
running on Linux because the kernel might schedule a process on another
hart.  There is no way for userspace to handle this without invoking the
kernel (as it doesn't know the thread->hart mappings), so we've defined
a RISC-V specific system call to flush the instruction cache.

This patch adds both a system call and a VDSO entry.  If possible, we'd
like to avoid having the system call be considered part of the
user-facing ABI and instead restrict that to the VDSO entry -- both just
in general to avoid having additional user-visible ABI to maintain, and
because we'd prefer that users just call the VDSO entry because there
might be a better way to do this in the future (ie, one that doesn't
require entering the kernel).
Signed-off-by: NAndrew Waterman <andrew@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

For now these are just placeholders that execute the syscall.  We will
later optimize them to avoid kernel crossings, but we'd like to have the
VDSO entries from the first released kernel version to make the ABI
simpler.
Signed-off-by: NAndrew Waterman <andrew@sifive.com>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

These were left over from an earlier version of the port.
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

This patch contains code that is in some way visible to the user:
including via system calls, the VDSO, module loading and signal
handling.  It also contains some generic code that is ABI visible.
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>