platform/kvm: map vdso and vvar into a guest address space

Right now, each vdso call triggers vmexit. VDSO and VVAR pages are
mapped with VM_IO and get_user_pages fails for such vma-s. KVM was not
able to handle this case up to the v4.8 kernel. This problem was fixed by
add6a0cd1c5ba ("KVM: MMU: try to fix up page faults before giving up").

For some unknown reasons, it still doesn't work in case of nested
virtualization.

Before:
BenchmarkKernelVDSO-6                   	  252519	      4598 ns/op
After:
BenchmarkKernelVDSO-6                   	34431957	        34.91 ns/op
PiperOrigin-RevId: 405715941

pkg/cpuid/cpuid_x86.go

@@ -81,7 +81,7 @@ const (
 	X86FeatureAVX
 	X86FeatureF16C
 	X86FeatureRDRAND
-	_ // ecx bit 31 is reserved.
+	X86FeatureHypervisor
 )
 
 // Block 1 constants are all of the "basic" feature bits returned by a cpuid in

pkg/sentry/platform/kvm/kvm_test.go

@@ -462,6 +462,22 @@ func TestRdtsc(t *testing.T) {
 	})
 }
 
+func TestKernelVDSO(t *testing.T) {
+	// Note that the target passed here is irrelevant, we never execute SwitchToUser.
+	applicationTest(t, true, testutil.AddrOfGetpid(), func(c *vCPU, regs *arch.Registers, pt *pagetables.PageTables) bool {
+		// iteration does not include machine.Get() / machine.Put().
+		const n = 100
+		for i := 0; i < n; i++ {
+			bluepill(c)
+			time.Now()
+		}
+		if c.guestExits >= n {
+			t.Errorf("vdso calls trigger vmexit")
+		}
+		return false
+	})
+}
+
 func BenchmarkApplicationSyscall(b *testing.B) {
 	var (
 		i int // Iteration includes machine.Get() / machine.Put().
@@ -498,6 +514,18 @@ func BenchmarkKernelSyscall(b *testing.B) {
 	})
 }
 
+func BenchmarkKernelVDSO(b *testing.B) {
+	// Note that the target passed here is irrelevant, we never execute SwitchToUser.
+	applicationTest(b, true, testutil.AddrOfGetpid(), func(c *vCPU, regs *arch.Registers, pt *pagetables.PageTables) bool {
+		// iteration does not include machine.Get() / machine.Put().
+		for i := 0; i < b.N; i++ {
+			bluepill(c)
+			time.Now()
+		}
+		return false
+	})
+}
+
 func BenchmarkWorldSwitchToUserRoundtrip(b *testing.B) {
 	// see BenchmarkApplicationSyscall.
 	var (

pkg/sentry/platform/kvm/machine_amd64.go

@@ -279,10 +279,13 @@ func (c *vCPU) fault(signal int32, info *linux.SignalInfo) (hostarch.AccessType,
 	// Reset the pointed SignalInfo.
 	*info = linux.SignalInfo{Signo: signal}
 	info.SetAddr(uint64(faultAddr))
-	accessType := hostarch.AccessType{
-		Read:    code&(1<<1) == 0,
-		Write:   code&(1<<1) != 0,
-		Execute: code&(1<<4) != 0,
+	accessType := hostarch.AccessType{}
+	if signal == int32(unix.SIGSEGV) {
+		accessType = hostarch.AccessType{
+			Read:    code&(1<<1) == 0,
+			Write:   code&(1<<1) != 0,
+			Execute: code&(1<<4) != 0,
+		}
 	}
 	if !accessType.Write && !accessType.Execute {
 		info.Code = 1 // SEGV_MAPERR.

pkg/sentry/platform/kvm/virtual_map.go

@@ -40,14 +40,9 @@ var mapsLine = regexp.MustCompile("([0-9a-f]+)-([0-9a-f]+) ([r-][w-][x-][sp]) ([
 // physical map. Virtual regions need to be excluded if get_user_pages will
 // fail on those addresses, preventing KVM from satisfying EPT faults.
 //
-// This includes the VVAR page because the VVAR page may be mapped as I/O
-// memory. And the VDSO page is knocked out because the VVAR page is not even
-// recorded in /proc/self/maps on older kernels; knocking out the VDSO page
-// prevents code in the VDSO from accessing the VVAR address.
-//
 // This is called by the physical map functions, not applyVirtualRegions.
 func excludeVirtualRegion(r virtualRegion) bool {
-	return r.filename == "[vvar]" || r.filename == "[vdso]"
+	return false
 }
 
 // applyVirtualRegions parses the process maps file.