riscv-qemu: add VirtIO RNG device

This implements machine.GetRNG() using VirtIO. This gets the tests to
pass for crypto/md5 and crypto/sha1 that use crypto/rand in their tests.

GNUmakefile

@@ -429,7 +429,6 @@ TEST_PACKAGES_NONWASM = \
 #
 #   * No filesystem is available, so packages like compress/zlib can't be tested
 #     (just like wasm).
-#   * There is no RNG implemented (TODO, I think this is fixable).
 #   * picolibc math functions apparently are less precise, the math package
 #     fails on baremetal.
 #   * Some packages fail or hang for an unknown reason, this should be
@@ -438,8 +437,6 @@ TEST_PACKAGES_BAREMETAL = $(filter-out $(TEST_PACKAGES_NONBAREMETAL), $(TEST_PAC
 TEST_PACKAGES_NONBAREMETAL = \
 	$(TEST_PACKAGES_NONWASM) \
 	crypto/elliptic \
-	crypto/md5 \
-	crypto/sha1 \
 	math \
 	reflect \
 	encoding/asn1 \

src/crypto/rand/rand_baremetal.go

@@ -1,4 +1,4 @@
-//go:build nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey
+//go:build nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt)
 
 // If you update the above build constraint, you'll probably also need to update
 // src/runtime/rand_hwrng.go.

src/machine/virt.go

@@ -0,0 +1,189 @@
+//go:build tinygo.riscv32 && virt
+
+// Machine implementation for VirtIO targets.
+// At the moment only QEMU RISC-V is supported, but support for ARM for example
+// should not be difficult to add with a change to virtioFindDevice.
+
+package machine
+
+import (
+	"errors"
+	"runtime/volatile"
+	"sync"
+	"unsafe"
+)
+
+const deviceName = "riscv-qemu"
+
+func (p Pin) Set(high bool) {
+	// no pins defined
+}
+
+var rngLock sync.Mutex
+var rngDevice *virtioDevice1
+var rngBuf volatile.Register32
+
+var errNoRNG = errors.New("machine: no entropy source found")
+var errNoRNGData = errors.New("machine: entropy source didn't return enough data")
+
+// GetRNG returns random numbers from a VirtIO entropy source.
+// When running in QEMU, it requires adding the RNG device:
+//
+//	-device virtio-rng-device
+func GetRNG() (uint32, error) {
+	rngLock.Lock()
+
+	// Initialize the device on first use.
+	if rngDevice == nil {
+		// Search for an available RNG.
+		rngDevice = virtioFindDevice(virtioDeviceEntropySource)
+		if rngDevice == nil {
+			rngLock.Unlock()
+			return 0, errNoRNG
+		}
+
+		// Initialize the device.
+		rngDevice.status.Set(0) // reset device
+		rngDevice.status.Set(virtioDeviceStatusAcknowledge)
+		rngDevice.status.Set(virtioDeviceStatusAcknowledge | virtioDeviceStatusDriver)
+		rngDevice.hostFeaturesSel.Set(0)
+		rngDevice.status.Set(virtioDeviceStatusAcknowledge | virtioDeviceStatusDriver | virtioDeviceStatusDriverOk)
+		rngDevice.guestPageSize.Set(4096)
+
+		// Configure queue, according to section 4.2.4 "Legacy interface".
+		// Note: we're skipping checks for queuePFM and queueNumMax.
+		rngDevice.queueSel.Set(0)      // use queue 0 (the only queue)
+		rngDevice.queueNum.Set(1)      // use a single buffer in the queue
+		rngDevice.queueAlign.Set(4096) // default alignment appears to be 4096
+		rngDevice.queuePFN.Set(uint32(uintptr(unsafe.Pointer(&rngQueue))) / 4096)
+
+		// Configure the only buffer in the queue (but don't increment
+		// rngQueue.available yet).
+		rngQueue.buffers[0].address = uint64(uintptr(unsafe.Pointer(&rngBuf)))
+		rngQueue.buffers[0].length = uint32(unsafe.Sizeof(rngBuf))
+		rngQueue.buffers[0].flags = 2 // 2 means write-only buffer
+	}
+
+	// Increment the available ring buffer. This doesn't actually change the
+	// buffer index (it's a ring with a single entry), but the number needs to
+	// be incremented otherwise the device won't recognize a new buffer.
+	index := rngQueue.available.index
+	rngQueue.available.index = index + 1
+	rngDevice.queueNotify.Set(0) // notify the device of the 'new' (reused) buffer
+	for rngQueue.used.index.Get() != index+1 {
+		// Busy wait until the RNG buffer is filled.
+		// A better way would be to wait for an interrupt, but since this driver
+		// implementation is mostly used for testing it's good enough for now.
+	}
+
+	// Check that we indeed got 4 bytes back.
+	if rngQueue.used.ring[0].length != 4 {
+		rngLock.Unlock()
+		return 0, errNoRNGData
+	}
+
+	// Read the resulting random numbers.
+	result := rngBuf.Get()
+
+	rngLock.Unlock()
+
+	return result, nil
+}
+
+// Implement a driver for the VirtIO entropy device.
+// https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html
+// http://wiki.osdev.org/Virtio
+// http://www.dumais.io/index.php?article=aca38a9a2b065b24dfa1dee728062a12
+
+const (
+	virtioDeviceStatusAcknowledge = 1
+	virtioDeviceStatusDriver      = 2
+	virtioDeviceStatusDriverOk    = 4
+	virtioDeviceStatusFeaturesOk  = 8
+	virtioDeviceStatusFailed      = 128
+)
+
+const (
+	virtioDeviceReserved = iota
+	virtioDeviceNetworkCard
+	virtioDeviceBlockDevice
+	virtioDeviceConsole
+	virtioDeviceEntropySource
+	// there are more device types
+)
+
+// VirtIO device version 1
+type virtioDevice1 struct {
+	magic            volatile.Register32 // always 0x74726976
+	version          volatile.Register32
+	deviceID         volatile.Register32
+	vendorID         volatile.Register32
+	hostFeatures     volatile.Register32
+	hostFeaturesSel  volatile.Register32
+	_                [2]uint32
+	guestFeatures    volatile.Register32
+	guestFeaturesSel volatile.Register32
+	guestPageSize    volatile.Register32
+	_                uint32
+	queueSel         volatile.Register32
+	queueNumMax      volatile.Register32
+	queueNum         volatile.Register32
+	queueAlign       volatile.Register32
+	queuePFN         volatile.Register32
+	_                [3]uint32
+	queueNotify      volatile.Register32
+	_                [3]uint32
+	interruptStatus  volatile.Register32
+	interruptAck     volatile.Register32
+	_                [2]uint32
+	status           volatile.Register32
+}
+
+// VirtIO queue, with a single buffer.
+type virtioQueue struct {
+	buffers [1]struct {
+		address uint64
+		length  uint32
+		flags   uint16
+		next    uint16
+	} // 16 bytes
+
+	available struct {
+		flags      uint16
+		index      uint16
+		ring       [1]uint16
+		eventIndex uint16
+	} // 8 bytes
+
+	_ [4096 - 16*1 - 8*1]byte // padding (to align on a 4096 byte boundary)
+
+	used struct {
+		flags uint16
+		index volatile.Register16
+		ring  [1]struct {
+			index  uint32
+			length uint32
+		}
+		availEvent uint16
+	}
+}
+
+func virtioFindDevice(deviceID uint32) *virtioDevice1 {
+	// On RISC-V, QEMU defines 8 VirtIO devices starting at 0x10001000 and
+	// repeating every 0x1000 bytes.
+	// The memory map can be seen in the QEMU source code:
+	// https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c
+	for i := 0; i < 8; i++ {
+		dev := (*virtioDevice1)(unsafe.Pointer(uintptr(0x10001000 + i*0x1000)))
+		if dev.magic.Get() != 0x74726976 || dev.version.Get() != 1 || dev.deviceID.Get() != deviceID {
+			continue
+		}
+		return dev
+	}
+	return nil
+}
+
+// A VirtIO queue needs to be page-aligned.
+//
+//go:align 4096
+var rngQueue virtioQueue

src/runtime/rand_hwrng.go

@@ -1,4 +1,4 @@
-//go:build baremetal && (nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey)
+//go:build baremetal && (nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt))
 
 // If you update the above build constraint, you'll probably also need to update
 // src/crypto/rand/rand_baremetal.go.

src/runtime/rand_norng.go

@@ -1,4 +1,4 @@
-//go:build baremetal && !(nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey)
+//go:build baremetal && !(nrf || (stm32 && !(stm32f103 || stm32l0x1)) || (sam && atsamd51) || (sam && atsame5x) || esp32c3 || tkey || (tinygo.riscv32 && virt))
 
 package runtime
 

targets/riscv-qemu.json

@@ -4,5 +4,5 @@
 	"build-tags": ["virt", "qemu"],
 	"default-stack-size": 4096,
 	"linkerscript": "targets/riscv-qemu.ld",
-	"emulator": "qemu-system-riscv32 -machine virt -nographic -bios none -kernel {}"
+	"emulator": "qemu-system-riscv32 -machine virt -nographic -bios none -device virtio-rng-device -kernel {}"
 }