KVM: arm64: guest debug, add support for single-step

This adds support for single-stepping the guest. To do this we need to
manipulate the guests PSTATE.SS and MDSCR_EL1.SS bits to trigger
stepping. We take care to preserve MDSCR_EL1 and trap access to it to
ensure we don't affect the apparent state of the guest.

As we have to enable trapping of all software debug exceptions we
suppress the ability of the guest to single-step itself. If we didn't we
would have to deal with the exception arriving while the guest was in
kernelspace when the guest is expecting to single-step userspace. This
is something we don't want to unwind in the kernel. Once the host is no
longer debugging the guest its ability to single-step userspace is
restored.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 27f38a9..e9de13e 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -103,6 +103,7 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	run->debug.arch.hsr = hsr;
 
 	switch (hsr >> ESR_ELx_EC_SHIFT) {
+	case ESR_ELx_EC_SOFTSTP_LOW:
 	case ESR_ELx_EC_BKPT32:
 	case ESR_ELx_EC_BRK64:
 		break;
@@ -130,6 +131,7 @@ static exit_handle_fn arm_exit_handlers[] = {
 	[ESR_ELx_EC_SYS64]	= kvm_handle_sys_reg,
 	[ESR_ELx_EC_IABT_LOW]	= kvm_handle_guest_abort,
 	[ESR_ELx_EC_DABT_LOW]	= kvm_handle_guest_abort,
+	[ESR_ELx_EC_SOFTSTP_LOW]= kvm_handle_guest_debug,
 	[ESR_ELx_EC_BKPT32]	= kvm_handle_guest_debug,
 	[ESR_ELx_EC_BRK64]	= kvm_handle_guest_debug,
 };