KVM: arm64: Don't hijack guest context MDSCR_EL1

Stealing MDSCR_EL1 in the guest's kvm_cpu_context for external debugging
is rather gross. Just add a field for this instead and let the context
switch code pick the correct one based on the debug owner.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20241219224116.3941496-15-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_host.h

@@ -751,6 +751,7 @@ struct kvm_vcpu_arch {
 	 */
 	struct kvm_guest_debug_arch vcpu_debug_state;
 	struct kvm_guest_debug_arch external_debug_state;
+	u64 external_mdscr_el1;
 
 	enum {
 		VCPU_DEBUG_FREE,
@@ -771,7 +772,6 @@ struct kvm_vcpu_arch {
 	 * are using guest debug.
 	 */
 	struct {
-		u32	mdscr_el1;
 		bool	pstate_ss;
 	} guest_debug_preserved;
 

arch/arm64/kvm/debug.c

@@ -31,19 +31,12 @@
  */
 static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
 {
-	u64 val = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
-
-	vcpu->arch.guest_debug_preserved.mdscr_el1 = val;
 	vcpu->arch.guest_debug_preserved.pstate_ss =
 					(*vcpu_cpsr(vcpu) & DBG_SPSR_SS);
 }
 
 static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
 {
-	u64 val = vcpu->arch.guest_debug_preserved.mdscr_el1;
-
-	vcpu_write_sys_reg(vcpu, val, MDSCR_EL1);
-
 	if (vcpu->arch.guest_debug_preserved.pstate_ss)
 		*vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
 	else
@@ -115,8 +108,6 @@ static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
 
 void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 {
-	unsigned long mdscr;
-
 	/* Check if we need to use the debug registers. */
 	if (vcpu->guest_debug || kvm_vcpu_os_lock_enabled(vcpu)) {
 		/* Save guest debug state */
@@ -154,36 +145,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
 				*vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
 			else
 				*vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
-
-			mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
-			mdscr |= DBG_MDSCR_SS;
-			vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
-		} else {
-			mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
-			mdscr &= ~DBG_MDSCR_SS;
-			vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
-		}
-
-		/*
-		 * Enable breakpoints and watchpoints if userspace wants them.
-		 */
-		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW) {
-			mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
-			mdscr |= DBG_MDSCR_MDE;
-			vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
-
-		/*
-		 * The OS Lock blocks debug exceptions in all ELs when it is
-		 * enabled. If the guest has enabled the OS Lock, constrain its
-		 * effects to the guest. Emulate the behavior by clearing
-		 * MDSCR_EL1.MDE. In so doing, we ensure that host debug
-		 * exceptions are unaffected by guest configuration of the OS
-		 * Lock.
-		 */
-		} else if (kvm_vcpu_os_lock_enabled(vcpu)) {
-			mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
-			mdscr &= ~DBG_MDSCR_MDE;
-			vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
 		}
 	}
 }
@@ -227,6 +188,41 @@ void kvm_init_host_debug_data(void)
 		host_data_set_flag(HAS_TRBE);
 }
 
+/*
+ * Configures the 'external' MDSCR_EL1 value for the guest, i.e. when the host
+ * has taken over MDSCR_EL1.
+ *
+ *  - Userspace is single-stepping the guest, and MDSCR_EL1.SS is forced to 1.
+ *
+ *  - Userspace is using the breakpoint/watchpoint registers to debug the
+ *    guest, and MDSCR_EL1.MDE is forced to 1.
+ *
+ *  - The guest has enabled the OS Lock, and KVM is forcing MDSCR_EL1.MDE to 0,
+ *    masking all debug exceptions affected by the OS Lock.
+ */
+static void setup_external_mdscr(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * Use the guest's MDSCR_EL1 as a starting point, since there are
+	 * several other features controlled by MDSCR_EL1 that are not relevant
+	 * to the host.
+	 *
+	 * Clear the bits that KVM may use which also satisfies emulation of
+	 * the OS Lock as MDSCR_EL1.MDE is cleared.
+	 */
+	u64 mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1) & ~(MDSCR_EL1_SS |
+							   MDSCR_EL1_MDE |
+							   MDSCR_EL1_KDE);
+
+	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
+		mdscr |= MDSCR_EL1_SS;
+
+	if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
+		mdscr |= MDSCR_EL1_MDE | MDSCR_EL1_KDE;
+
+	vcpu->arch.external_mdscr_el1 = mdscr;
+}
+
 void kvm_vcpu_load_debug(struct kvm_vcpu *vcpu)
 {
 	u64 mdscr;
@@ -249,6 +245,7 @@ void kvm_vcpu_load_debug(struct kvm_vcpu *vcpu)
 	 */
 	if (vcpu->guest_debug || kvm_vcpu_os_lock_enabled(vcpu)) {
 		vcpu->arch.debug_owner = VCPU_DEBUG_HOST_OWNED;
+		setup_external_mdscr(vcpu);
 	} else {
 		mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
 

arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h

@@ -18,9 +18,34 @@
 
 static inline bool ctxt_has_s1poe(struct kvm_cpu_context *ctxt);
 
+static inline struct kvm_vcpu *ctxt_to_vcpu(struct kvm_cpu_context *ctxt)
+{
+	struct kvm_vcpu *vcpu = ctxt->__hyp_running_vcpu;
+
+	if (!vcpu)
+		vcpu = container_of(ctxt, struct kvm_vcpu, arch.ctxt);
+
+	return vcpu;
+}
+
+static inline bool ctxt_is_guest(struct kvm_cpu_context *ctxt)
+{
+	return host_data_ptr(host_ctxt) != ctxt;
+}
+
+static inline u64 *ctxt_mdscr_el1(struct kvm_cpu_context *ctxt)
+{
+	struct kvm_vcpu *vcpu = ctxt_to_vcpu(ctxt);
+
+	if (ctxt_is_guest(ctxt) && kvm_host_owns_debug_regs(vcpu))
+		return &vcpu->arch.external_mdscr_el1;
+
+	return &ctxt_sys_reg(ctxt, MDSCR_EL1);
+}
+
 static inline void __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
 {
-	ctxt_sys_reg(ctxt, MDSCR_EL1)	= read_sysreg(mdscr_el1);
+	*ctxt_mdscr_el1(ctxt)	= read_sysreg(mdscr_el1);
 
 	// POR_EL0 can affect uaccess, so must be saved/restored early.
 	if (ctxt_has_s1poe(ctxt))
@@ -33,16 +58,6 @@ static inline void __sysreg_save_user_state(struct kvm_cpu_context *ctxt)
 	ctxt_sys_reg(ctxt, TPIDRRO_EL0)	= read_sysreg(tpidrro_el0);
 }
 
-static inline struct kvm_vcpu *ctxt_to_vcpu(struct kvm_cpu_context *ctxt)
-{
-	struct kvm_vcpu *vcpu = ctxt->__hyp_running_vcpu;
-
-	if (!vcpu)
-		vcpu = container_of(ctxt, struct kvm_vcpu, arch.ctxt);
-
-	return vcpu;
-}
-
 static inline bool ctxt_has_mte(struct kvm_cpu_context *ctxt)
 {
 	struct kvm_vcpu *vcpu = ctxt_to_vcpu(ctxt);
@@ -139,7 +154,7 @@ static inline void __sysreg_save_el2_return_state(struct kvm_cpu_context *ctxt)
 
 static inline void __sysreg_restore_common_state(struct kvm_cpu_context *ctxt)
 {
-	write_sysreg(ctxt_sys_reg(ctxt, MDSCR_EL1),  mdscr_el1);
+	write_sysreg(*ctxt_mdscr_el1(ctxt),  mdscr_el1);
 
 	// POR_EL0 can affect uaccess, so must be saved/restored early.
 	if (ctxt_has_s1poe(ctxt))