apic: add read() method

arch/x64/apic.cc

@@ -7,6 +7,7 @@ class x2apic : public apic_driver {
 public:
     explicit x2apic();
     virtual void init_on_ap();
+    virtual u32 read(apicreg reg);
     virtual void write(apicreg reg, u32 value);
     virtual void self_ipi(unsigned vector);
     virtual void ipi(unsigned apic_id, unsigned vector);
@@ -75,6 +76,11 @@ void x2apic::eoi()
     wrmsr(msr::X2APIC_EOI, 0);
 }
 
+u32 x2apic::read(apicreg reg)
+{
+    return processor::rdmsr(0x800 + unsigned(reg) / 0x10);
+}
+
 void x2apic::write(apicreg reg, u32 value)
 {
     processor::wrmsr(0x800 + unsigned(reg) / 0x10, value);

arch/x64/apic.hh

@@ -83,6 +83,7 @@ public:
     virtual void ipi_allbutself(unsigned vector) = 0;
     virtual void nmi_allbutself() = 0;
     virtual void eoi() = 0;
+    virtual u32 read(apicreg reg) = 0;
     virtual void write(apicreg reg, u32 value) = 0;
     virtual u32 id() = 0;
     void set_lvt(apiclvt reg, unsigned vector);