[PATCH 1/2] KVM: arm64: Flush external_mdscr_el1 to the pKVM hyp vCPU

[Fuad Tabba <fuad.tabba@linux.dev>]

flush_debug_state() propagates the guest's debug_owner and the
owner-selected debug register state to the hyp vCPU, but not
external_mdscr_el1. While the host owns the debug registers, the world
switch loads MDSCR_EL1 from external_mdscr_el1 (ctxt_mdscr_el1()),
where the host's KDE/MDE/SS bits live. A non-protected guest under
KVM_GUESTDBG_USE_HW or single-step therefore runs with MDSCR_EL1.MDE/SS
clear in hardware, and its watchpoints, breakpoints and single-step
never fire.

Propagate external_mdscr_el1 to the hyp vCPU alongside the host-owned
debug state.

Fixes: 4ad3a0b87f2ec ("KVM: arm64: Don't hijack guest context MDSCR_EL1")
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index d3c69de698f48..104026ee70e80 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -108,10 +108,16 @@ static void flush_debug_state(struct pkvm_hyp_vcpu *hyp_vcpu)
 
 	hyp_vcpu->vcpu.arch.debug_owner = host_vcpu->arch.debug_owner;
 
-	if (kvm_guest_owns_debug_regs(&hyp_vcpu->vcpu))
+	if (kvm_guest_owns_debug_regs(&hyp_vcpu->vcpu)) {
 		hyp_vcpu->vcpu.arch.vcpu_debug_state = host_vcpu->arch.vcpu_debug_state;
-	else if (kvm_host_owns_debug_regs(&hyp_vcpu->vcpu))
+	} else if (kvm_host_owns_debug_regs(&hyp_vcpu->vcpu)) {
 		hyp_vcpu->vcpu.arch.external_debug_state = host_vcpu->arch.external_debug_state;
+		/*
+		 * The world switch loads MDSCR_EL1 from external_mdscr_el1
+		 * (ctxt_mdscr_el1()).
+		 */
+		hyp_vcpu->vcpu.arch.external_mdscr_el1 = host_vcpu->arch.external_mdscr_el1;
+	}
 }
 
 static void sync_debug_state(struct pkvm_hyp_vcpu *hyp_vcpu)
-- 
2.39.5