Implement virtio-rng device

This commit introduces the VirtIO entropy device, also known as virtio-rng in
QEMU or the Linux kernel.

Randomness is a precious resource in the system. Without sufficient entropy,
functions like arc4random_buf() in the standard C library may not work properly
since they rely on the blocking device /dev/random.

Quaoted from https://en.wikipedia.org/wiki//dev/random

/dev/random typically blocked if there was less entropy available than
requested; more recently (see below for the differences between operating
systems) it usually blocks at startup until sufficient entropy has been
gathered, then unblocks permanently.

With Linux kernel 3.16 and newer, the kernel itself mixes data from hardware
random number generators into /dev/random on a sliding scale based on the
definable entropy estimation quality of the HWRNG. This means that no userspace
daemon, such as rngd from rng-tools, is needed to do that job. With Linux
kernel 3.17+, the VirtIO RNG was modified to have a default quality defined
above 0, and as such, is currently the only HWRNG mixed into /dev/random by
default.

Close #68.

Makefile

@@ -31,6 +31,13 @@ ifeq ($(call has, VIRTIOBLK), 1)
     endif
 endif
 
+# virtio-rng
+ENABLE_VIRTIORNG ?= 1
+$(call set-feature, VIRTIORNG)
+ifeq ($(call has, VIRTIORNG), 1)
+    OBJS_EXTRA += virtio-rng.o
+endif
+
 NETDEV ?= tap
 # virtio-net
 ENABLE_VIRTIONET ?= 1

device.h

@@ -172,6 +172,52 @@ void virtio_blk_write(hart_t *vm,
 uint32_t *virtio_blk_init(virtio_blk_state_t *vblk, char *disk_file);
 #endif /* SEMU_HAS(VIRTIOBLK) */
 
+/* VirtIO-RNG */
+
+#if SEMU_HAS(VIRTIORNG)
+
+#define IRQ_VRNG 4
+#define IRQ_VRNG_BIT (1 << IRQ_VRNG)
+
+typedef struct {
+    uint32_t QueueNum;
+    uint32_t QueueDesc;
+    uint32_t QueueAvail;
+    uint32_t QueueUsed;
+    uint16_t last_avail;
+    bool ready;
+} virtio_rng_queue_t;
+
+typedef struct {
+    /* feature negotiation */
+    uint32_t DeviceFeaturesSel;
+    uint32_t DriverFeatures;
+    uint32_t DriverFeaturesSel;
+    /* queue config */
+    uint32_t QueueSel;
+    virtio_rng_queue_t queues[1];
+    /* status */
+    uint32_t Status;
+    uint32_t InterruptStatus;
+    /* supplied by environment */
+    uint32_t *ram;
+} virtio_rng_state_t;
+
+void virtio_rng_read(hart_t *vm,
+                     virtio_rng_state_t *rng,
+                     uint32_t addr,
+                     uint8_t width,
+                     uint32_t *value);
+
+void virtio_rng_write(hart_t *vm,
+                      virtio_rng_state_t *vrng,
+                      uint32_t addr,
+                      uint8_t width,
+                      uint32_t value);
+
+void virtio_rng_init(void);
+#endif /* SEMU_HAS(VIRTIORNG) */
+
 /* ACLINT MTIMER */
 typedef struct {
     /* A MTIMER device has two separate base addresses: one for the MTIME
@@ -272,6 +318,9 @@ typedef struct {
 #endif
 #if SEMU_HAS(VIRTIOBLK)
     virtio_blk_state_t vblk;
+#endif
+#if SEMU_HAS(VIRTIORNG)
+    virtio_rng_state_t vrng;
 #endif
     /* ACLINT */
     mtimer_state_t mtimer;

main.c

@@ -72,6 +72,18 @@ static void emu_update_vblk_interrupts(vm_t *vm)
 }
 #endif
 
+#if SEMU_HAS(VIRTIORNG)
+static void emu_update_vrng_interrupts(vm_t *vm)
+{
+    emu_state_t *data = PRIV(vm->hart[0]);
+    if (data->vrng.InterruptStatus)
+        data->plic.active |= IRQ_VRNG_BIT;
+    else
+        data->plic.active &= ~IRQ_VRNG_BIT;
+    plic_update_interrupts(vm, &data->plic);
+}
+#endif
+
 static void emu_update_timer_interrupt(hart_t *hart)
 {
     emu_state_t *data = PRIV(hart);
@@ -137,6 +149,12 @@ static void mem_load(hart_t *hart,
             aclint_sswi_read(hart, &data->sswi, addr & 0xFFFFF, width, value);
             aclint_sswi_update_interrupts(hart, &data->sswi);
             return;
+#if SEMU_HAS(VIRTIORNG)
+        case 0x46: /* virtio-rng */
+            virtio_rng_read(hart, &data->vrng, addr & 0xFFFFF, width, value);
+            emu_update_vrng_interrupts(hart->vm);
+            return;
+#endif
         }
     }
     vm_set_exception(hart, RV_EXC_LOAD_FAULT, hart->exc_val);
@@ -191,6 +209,12 @@ static void mem_store(hart_t *hart,
             aclint_sswi_write(hart, &data->sswi, addr & 0xFFFFF, width, value);
             aclint_sswi_update_interrupts(hart, &data->sswi);
             return;
+#if SEMU_HAS(VIRTIORNG)
+        case 0x46: /* virtio-rng */
+            virtio_rng_write(hart, &data->vrng, addr & 0xFFFFF, width, value);
+            emu_update_vrng_interrupts(hart->vm);
+            return;
+#endif
         }
     }
     vm_set_exception(hart, RV_EXC_STORE_FAULT, hart->exc_val);
@@ -617,6 +641,10 @@ static int semu_start(int argc, char **argv)
 #if SEMU_HAS(VIRTIOBLK)
     emu.vblk.ram = emu.ram;
     emu.disk = virtio_blk_init(&(emu.vblk), disk_file);
+#endif
+#if SEMU_HAS(VIRTIORNG)
+    emu.vrng.ram = emu.ram;
+    virtio_rng_init();
 #endif
     /* Set up ACLINT */
     semu_timer_init(&emu.mtimer.mtime, CLOCK_FREQ);

minimal.dts

@@ -63,5 +63,13 @@
             interrupts = <3>;
         };
 #endif
+
+#if SEMU_FEATURE_VIRTIORNG
+        rng0: virtio@4600000 {
+            compatible = "virtio,mmio";
+            reg = <0x4600000 0x200>;
+            interrupts = <4>;
+        };
+#endif
     };
 };