ring0: set PhysicalAddressBits only when uninitialized

Currently ring0.PhysicalAddressBits will be set in ring0.Init() even
if it has already been initialized elsewhere. For example, when
compiling runsc with cgo enabled, function init() of the package
kvm(machine_cgo.go) sets this variable. And this value would be updated
by ring0.Init() when creating a new KVM context. If two values are
inconsistent, the sandbox would panic during later memory region
setting.

This patch fixes it by checking the PhysicalAddressBits in ring0.Init()
and setting the value only if it is uninitialized.

Signed-off-by: Tianyu Zhou <[email protected]>

pkg/ring0/lib_amd64.go

@@ -106,7 +106,9 @@ func Init(fs cpuid.FeatureSet) {
 	if VirtualAddressBits > 48 {
 		VirtualAddressBits = 48
 	}
-	PhysicalAddressBits = uintptr(fs.PhysicalAddressBits())
+	if PhysicalAddressBits == 0 {
+		PhysicalAddressBits = uintptr(fs.PhysicalAddressBits())
+	}
 	UserspaceSize = uintptr(1) << (VirtualAddressBits - 1)
 	MaximumUserAddress = (UserspaceSize - 1) & ^uintptr(hostarch.PageSize-1)
 	KernelStartAddress = ^uintptr(0) - (UserspaceSize - 1)