platform/ptrace: use host.GetCPU instead of the getcpu syscall

This should save ~200ns from switchToApp (on ptrace too). // mpratt

PiperOrigin-RevId: 281159895

pkg/sentry/platform/ptrace/BUILD

@@ -28,6 +28,7 @@ go_library(
         "//pkg/procid",
         "//pkg/seccomp",
         "//pkg/sentry/arch",
+        "//pkg/sentry/hostcpu",
         "//pkg/sentry/platform",
         "//pkg/sentry/platform/interrupt",
         "//pkg/sentry/platform/safecopy",

pkg/sentry/platform/ptrace/subprocess_linux_unsafe.go

@@ -25,6 +25,7 @@ import (
 
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/hostcpu"
 )
 
 // maskPool contains reusable CPU masks for setting affinity. Unfortunately,
@@ -49,20 +50,6 @@ func unmaskAllSignals() syscall.Errno {
 	return errno
 }
 
-// getCPU gets the current CPU.
-//
-// Precondition: the current runtime thread should be locked.
-func getCPU() (uint32, error) {
-	var cpu uintptr
-	if _, _, errno := syscall.RawSyscall(
-		unix.SYS_GETCPU,
-		uintptr(unsafe.Pointer(&cpu)),
-		0, 0); errno != 0 {
-		return 0, errno
-	}
-	return uint32(cpu), nil
-}
-
 // setCPU sets the CPU affinity.
 func (t *thread) setCPU(cpu uint32) error {
 	mask := maskPool.Get().([]uintptr)
@@ -93,10 +80,8 @@ func (t *thread) setCPU(cpu uint32) error {
 //
 // Precondition: the current runtime thread should be locked.
 func (t *thread) bind() {
-	currentCPU, err := getCPU()
-	if err != nil {
-		return
-	}
+	currentCPU := hostcpu.GetCPU()
+
 	if oldCPU := atomic.SwapUint32(&t.cpu, currentCPU); oldCPU != currentCPU {
 		// Set the affinity on the thread and save the CPU for next
 		// round; we don't expect CPUs to bounce around too frequently.