[RFC][PATCH 3/4] kvm-vmx: KVM_EXIT_DEBUG on #BP exceptions

[Jan Kiszka <jan.kiszka@web.de>]

In order to allow the gdbstub of QEMU to push (soft) breakpoint handling
completely into the gdb frontend, this patch enables guest exits also
for #BP exceptions - in case guest debugging was turned on.

Along this enhancement, this patch also fixes the flag manipulation for
the singlestep mode.

---
 arch/x86/kvm/vmx.c |   38 +++++++++++++++-----------------------
 1 file changed, 15 insertions(+), 23 deletions(-)

Index: b/arch/x86/kvm/vmx.c
===================================================================
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -461,7 +461,7 @@ static void update_exception_bitmap(stru
 	if (!vcpu->fpu_active)
 		eb |= 1u << NM_VECTOR;
 	if (vcpu->guest_debug.enabled)
-		eb |= 1u << 1;
+		eb |= (1u << 1) | (1u << 3);
 	if (vcpu->arch.rmode.active)
 		eb = ~0;
 	if (vm_need_ept())
@@ -949,6 +949,7 @@ static int set_guest_debug(struct kvm_vc
 {
 	unsigned long dr7 = 0x400;
 	int old_singlestep;
+	unsigned long flags;
 
 	old_singlestep = vcpu->guest_debug.singlestep;
 
@@ -969,13 +970,12 @@ static int set_guest_debug(struct kvm_vc
 	} else
 		vcpu->guest_debug.singlestep = 0;
 
-	if (old_singlestep && !vcpu->guest_debug.singlestep) {
-		unsigned long flags;
-
-		flags = vmcs_readl(GUEST_RFLAGS);
+	flags = vmcs_readl(GUEST_RFLAGS);
+	if (vcpu->guest_debug.singlestep)
+		flags |= X86_EFLAGS_TF | X86_EFLAGS_RF;
+	else if (old_singlestep && !vcpu->guest_debug.singlestep)
 		flags &= ~(X86_EFLAGS_TF | X86_EFLAGS_RF);
-		vmcs_writel(GUEST_RFLAGS, flags);
-	}
+	vmcs_writel(GUEST_RFLAGS, flags);
 
 	update_exception_bitmap(vcpu);
 	vmcs_writel(GUEST_DR7, dr7);
@@ -2192,14 +2192,6 @@ static void kvm_guest_debug_pre(struct k
 	set_debugreg(dbg->bp[1], 1);
 	set_debugreg(dbg->bp[2], 2);
 	set_debugreg(dbg->bp[3], 3);
-
-	if (dbg->singlestep) {
-		unsigned long flags;
-
-		flags = vmcs_readl(GUEST_RFLAGS);
-		flags |= X86_EFLAGS_TF | X86_EFLAGS_RF;
-		vmcs_writel(GUEST_RFLAGS, flags);
-	}
 }
 
 static int handle_rmode_exception(struct kvm_vcpu *vcpu,
@@ -2221,7 +2213,7 @@ static int handle_rmode_exception(struct
 static int handle_exception(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	u32 intr_info, error_code;
+	u32 intr_info, ex_no, error_code;
 	unsigned long cr2, rip;
 	u32 vect_info;
 	enum emulation_result er;
@@ -2279,15 +2271,15 @@ static int handle_exception(struct kvm_v
 		return 1;
 	}
 
-	if ((intr_info & (INTR_INFO_INTR_TYPE_MASK | INTR_INFO_VECTOR_MASK)) ==
-	    (INTR_TYPE_EXCEPTION | 1)) {
+	ex_no = intr_info & INTR_INFO_VECTOR_MASK;
+	if (ex_no == 1 || ex_no == 3) {
 		kvm_run->exit_reason = KVM_EXIT_DEBUG;
-		kvm_run->debug.arch.exception = 1;
-		return 0;
+		kvm_run->debug.arch.exception = ex_no;
+	} else {
+		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
+		kvm_run->ex.exception = ex_no;
+		kvm_run->ex.error_code = error_code;
 	}
-	kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
-	kvm_run->ex.exception = intr_info & INTR_INFO_VECTOR_MASK;
-	kvm_run->ex.error_code = error_code;
 	return 0;
 }
 

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