KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit

Bus lock debug exception introduces a new bit DR6_BUS_LOCK (bit 11 of
DR6) to indicate that bus lock #DB exception is generated. The set/clear
of DR6_BUS_LOCK is similar to the DR6_RTM. The processor clears
DR6_BUS_LOCK when the exception is generated. For all other #DB, the
processor sets this bit to 1. Software #DB handler should set this bit
before returning to the interrupted task.

In VMM, to avoid breaking the CPUs without bus lock #DB exception
support, activate the DR6_BUS_LOCK conditionally in DR6_FIXED_1 bits.
When intercepting the #DB exception caused by bus locks, bit 11 of the
exit qualification is set to identify it. The VMM should emulate the
exception by clearing the bit 11 of the guest DR6.

Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20210202090433.13441-3-chenyi.qiang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -200,6 +200,7 @@ enum x86_intercept_stage;
 
 #define KVM_NR_DB_REGS	4
 
+#define DR6_BUS_LOCK   (1 << 11)
 #define DR6_BD		(1 << 13)
 #define DR6_BS		(1 << 14)
 #define DR6_BT		(1 << 15)
@@ -213,7 +214,7 @@ enum x86_intercept_stage;
  * DR6_ACTIVE_LOW is also used as the init/reset value for DR6.
  */
 #define DR6_ACTIVE_LOW	0xffff0ff0
-#define DR6_VOLATILE	0x0001e00f
+#define DR6_VOLATILE	0x0001e80f
 #define DR6_FIXED_1	(DR6_ACTIVE_LOW & ~DR6_VOLATILE)
 
 #define DR7_BP_EN_MASK	0x000000ff

arch/x86/kvm/x86.c

@@ -1176,6 +1176,9 @@ static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_RTM))
 		fixed |= DR6_RTM;
+
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
+		fixed |= DR6_BUS_LOCK;
 	return fixed;
 }