arm64: KVM: Add ARCH_WORKAROUND_2 support for guests

arm64: KVM: Add ARCH_WORKAROUND_2 support for guests

BugLink: https://bugs.launchpad.net/bugs/1787993
CVE-2018-3639 (arm64)

commit 55e3748e8902ff641e334226bdcb432f9a5d78d3 upstream.

In order to offer ARCH_WORKAROUND_2 support to guests, we need
a bit of infrastructure.

Let's add a flag indicating whether or not the guest uses
SSBD mitigation. Depending on the state of this flag, allow
KVM to disable ARCH_WORKAROUND_2 before entering the guest,
and enable it when exiting it.

Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 805357aa65bbc419451fb9556968687c7fd1d2e1
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git /
linux-4.14.y)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>