Re: [PATCH v4] powerpc/kvm: support to handle sw breakpoint

This patch adds kernel side support for software breakpoint.
Design is that, by using an illegal instruction, we trap to hypervisor
via Emulation Assistance interrupt, where we check for the illegal instruction
and accordingly we return to Host or Guest. Patch also adds support for
software breakpoint in PR KVM.

Changes v3->v4:
 Made changes to code comments and removed #define of zero opcode
 Added a new function to handle the debug instruction emulation in book3s_hv
 Rebased the code to latest upstream source.

Changes v2->v3:
 Changed the debug instructions. Using the all zero opcode in the instruction word
  as illegal instruction as mentioned in Power ISA instead of ABS
 Removed reg updated in emulation assist and added a call to
  kvmppc_emulate_instruction for reg update.

Changes v1->v2:

 Moved the debug instruction #def to kvm_book3s.h. This way PR_KVM can also share it.
 Added code to use KVM get one reg infrastructure to get debug opcode.
 Updated emulate.c to include emulation of debug instruction incase of PR_KVM.
 Made changes to commit message.

Signed-off-by: Madhavan Srinivasan <redacted>
---
 arch/powerpc/include/asm/kvm_book3s.h |  7 +++++++
 arch/powerpc/kvm/book3s.c             |  3 ++-
 arch/powerpc/kvm/book3s_hv.c          | 32 ++++++++++++++++++++++++++++++--
 arch/powerpc/kvm/book3s_pr.c          |  3 +++
 arch/powerpc/kvm/emulate.c            | 11 +++++++++++
 5 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
index 6acf0c2..a1944f8 100644
--- a/arch/powerpc/include/asm/kvm_book3s.h
+++ b/arch/powerpc/include/asm/kvm_book3s.h

@@ -24,6 +24,13 @@
 #include <linux/kvm_host.h>
 #include <asm/kvm_book3s_asm.h>
 
+/*
+ * KVMPPC_INST_BOOK3S_DEBUG is debug Instruction for supporting Software Breakpoint.
+ * Based on PowerISA v2.07, Instruction with primary opcode 0 will be treated as illegal
+ * instruction.
+ */
+#define KVMPPC_INST_BOOK3S_DEBUG	0x00dddd00

+
 struct kvmppc_bat {
 	u64 raw;
 	u32 bepi;

diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index dd03f6b..00e9c9f 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c

@@ -778,7 +778,8 @@ int kvm_arch_vcpu_ioctl_translate(struct kvm_vcpu *vcpu,
 int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 					struct kvm_guest_debug *dbg)
 {
-	return -EINVAL;
+	vcpu->guest_debug = dbg->control;
+	return 0;
 }
 
 void kvmppc_decrementer_func(unsigned long data)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 27cced9..0a92e45 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -725,6 +725,14 @@ static int kvmppc_hcall_impl_hv(unsigned long cmd)
 	return kvmppc_hcall_impl_hv_realmode(cmd);
 }
 
+static int kvmppc_emulate_debug_instruction_hv(struct kvm_run *run,
+					struct kvm_vcpu *vcpu)
+{
+	run->exit_reason = KVM_EXIT_DEBUG;
+	run->debug.arch.address = kvmppc_get_pc(vcpu);
+	return 0;
+}
+
 static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
 				 struct task_struct *tsk)
 {

@@ -811,9 +819,26 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	 * we don't emulate any guest instructions at this stage.

+			/*
+			 * Fetch failed, so return to guest and
+			 * try executing it again.
+			 */
+			r = RESUME_GUEST;
+		} else {
+			if (last_inst == KVMPPC_INST_BOOK3S_DEBUG) {
+				kvmppc_emulate_debug_instruction_hv(run, vcpu);
+				r = RESUME_HOST;
+			} else {
+				kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
+				r = RESUME_GUEST;
+			}
+		}
 		break;
+	}
 	/*
 	 * This occurs if the guest (kernel or userspace), does something that
 	 * is prohibited by HFSCR.  We just generate a program interrupt to

@@ -922,6 +947,9 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
 	long int i;
 
 	switch (id) {
+	case KVM_REG_PPC_DEBUG_INST:
+		*val = get_reg_val(id, KVMPPC_INST_BOOK3S_DEBUG);
+		break;
 	case KVM_REG_PPC_HIOR:
 		*val = get_reg_val(id, 0);
 		break;

diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
index faffb27..cd2a39d 100644
--- a/arch/powerpc/kvm/book3s_pr.c
+++ b/arch/powerpc/kvm/book3s_pr.c

@@ -1319,6 +1319,9 @@ static int kvmppc_get_one_reg_pr(struct kvm_vcpu *vcpu, u64 id,
 	int r = 0;
 
 	switch (id) {
+	case KVM_REG_PPC_DEBUG_INST:
+		*val = get_reg_val(id, KVMPPC_INST_BOOK3S_DEBUG);
+		break;
 	case KVM_REG_PPC_HIOR:
 		*val = get_reg_val(id, to_book3s(vcpu)->hior);
 		break;

diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
index e96b50d..68f38b2 100644
--- a/arch/powerpc/kvm/emulate.c
+++ b/arch/powerpc/kvm/emulate.c

@@ -274,6 +274,17 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
 		}
 		break;
 
+	case 0:
+		/*
+		 * Instruction with primary opcode 0. Based on PowerISA
+		 * these are illegal instructions.
+		 */
+		run->exit_reason = KVM_EXIT_DEBUG;