kvm: limit number of vCPU-s

We see a regression of the startup benchmark. The main reason is that the
maximum number of vCPUs in the kernel has been increased from 256 to 1024. In
most cases, we don't need so many vCPU-s.

PiperOrigin-RevId: 462455491

pkg/sentry/platform/kvm/machine_amd64.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"math/big"
 	"reflect"
+	"runtime"
 	"runtime/debug"
 
 	"golang.org/x/sys/unix"
@@ -478,6 +479,16 @@ func (m *machine) getMaxVCPU() {
 	} else {
 		m.maxVCPUs = int(maxVCPUs)
 	}
+
+	// The goal here is to avoid vCPU contentions for reasonable workloads.
+	// But "reasonable" isn't defined well in this case. Let's say that CPU
+	// overcommit with factor 2 is still acceptable. We allocate a set of
+	// vCPU for each goruntime processor (P) and two sets of vCPUs to run
+	// user code.
+	rCPUs := runtime.GOMAXPROCS(0)
+	if 3*rCPUs < m.maxVCPUs {
+		m.maxVCPUs = 3 * rCPUs
+	}
 }
 
 func archPhysicalRegions(physicalRegions []physicalRegion) []physicalRegion {