Implement VirtIO sound device playback

The limitations are listed as follows:

1. The emulator will hang if PulseAudio is enabled on host.
The reason is that the host OS cannot close CNFA driver
because CNFA driver reference count is not zero (PulseAudio
holds one reference count). What's worse, CNFA cannot initialize if PulseAudio
is disabled, As it needs a dedicated threading model (CNFA uses
different threading models between pure ALSA and PulseAudio environment),
we suggest users need mitigations (for instance, restart the emulator after
playing sound) or just wait for the future release.

2. The playback may play with repeating artifact (for example,
A "front center" ALSA example sound will sound like "front front
center"). The root cause is the Linux Kernel and it hasn't got
fixed even in mainline version. See
https://lore.kernel.org/all/[email protected]/T/ for more
information.

Author: Cuda-Chen

.github/workflows/main.yml

@@ -10,6 +10,7 @@ jobs:
     - name: install-dependencies
       run: |
             sudo apt-get install build-essential device-tree-compiler expect
+            sudo apt-get install libasound2-dev libudev-dev
     - name: default build
       run: make
       shell: bash

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "cnfa"]
+    path = cnfa
+    url = https://github.com/cntools/cnfa
+    shallow = true

Makefile

@@ -1,4 +1,5 @@
 include mk/common.mk
+include mk/check-libs.mk
 
 CC ?= gcc
 CFLAGS := -O2 -g -Wall -Wextra
@@ -13,6 +14,8 @@ OBJS_EXTRA :=
 # command line option
 OPTS :=
 
+LDFLAGS := -lm
+
 # virtio-blk
 ENABLE_VIRTIOBLK ?= 1
 $(call set-feature, VIRTIOBLK)
@@ -50,6 +53,43 @@ ifeq ($(call has, VIRTIONET), 1)
     OBJS_EXTRA += netdev.o
 endif
 
+# virtio-snd
+ENABLE_VIRTIOSND ?= 1
+ifneq ($(UNAME_S),$(filter $(UNAME_S),Linux))
+    ENABLE_VIRTIOSND := 0
+endif
+
+# Check ALSA installation
+ifeq ($(UNAME_S),Linux)
+    ifeq (0, $(call check-alsa))
+        $(warning No libasound installed. Check libasound in advance.)
+        ENABLE_VIRTIOSND := 0
+    endif
+endif
+$(call set-feature, VIRTIOSND)
+ifeq ($(call has, VIRTIOSND), 1)
+    OBJS_EXTRA += virtio-snd.o
+
+    LDFLAGS += -lasound -lpthread
+    CFLAGS += -Icnfa
+
+cnfa/Makefile:
+	git submodule update --init cnfa
+cnfa/os_generic: cnfa/Makefile
+	$(MAKE) -C $(dir $<) os_generic.h
+CNFA_LIB := cnfa/CNFA_sf.h
+$(CNFA_LIB): cnfa/Makefile cnfa/os_generic
+	$(MAKE) -C $(dir $<) CNFA_sf.h
+main.o: $(CNFA_LIB)
+
+# suppress warning when compiling CNFA
+virtio-snd.o: CFLAGS += -Wno-unused-parameter -Wno-sign-compare
+endif
+
+# .DEFAULT_GOAL should be set to all since the very first target is not all
+# after git submodule.
+.DEFAULT_GOAL := all
+
 BIN = semu
 all: $(BIN) minimal.dtb
 

README.md

@@ -9,7 +9,12 @@ A minimalist RISC-V system emulator capable of running Linux the kernel and corr
 - UART: 8250/16550
 - PLIC (platform-level interrupt controller): 32 interrupts, no priority
 - Standard SBI, with the timer extension
-- VirtIO: virtio-blk acquires disk image from the host, and virtio-net is mapped as TAP interface
+- Three types of I/O support using VirtIO standard:
+    - virtio-blk acquires disk image from the host.
+    - virtio-net is mapped as TAP interface.
+    - virtio-snd uses ALSA for sound operation with the following limitations:
+        - The emulator will hang if PulseAudio is enabled on host.
+        - The playback may plays with repeating artifact.
 
 ## Prerequisites
 

cnfa

@@ -36,6 +36,9 @@ BR2_RELRO_NONE=y
 # BR2_RELRO_PARTIAL is not set
 # BR2_RELRO_FULL is not set
 BR2_FORTIFY_SOURCE_1=y
+BR2_PACKAGE_ALSA_UTILS=y
+BR2_PACKAGE_ALSA_UTILS_APLAY=y
+BR2_PACKAGE_ALSA_UTILS_SPEAKER_TEST=y
 # BR2_PACKAGE_URANDOM_SCRIPTS is not set
 BR2_TARGET_ROOTFS_CPIO=y
 BR2_TARGET_ROOTFS_CPIO_FULL=y

configs/buildroot.config

@@ -30,7 +30,8 @@ CONFIG_LOCALVERSION_AUTO=y
 CONFIG_BUILD_SALT=""
 CONFIG_DEFAULT_INIT=""
 CONFIG_DEFAULT_HOSTNAME="(none)"
-# CONFIG_SYSVIPC is not set
+CONFIG_SYSVIPC=y
+CONFIG_SYSVIPC_SYSCTL=y
 # CONFIG_POSIX_MQUEUE is not set
 # CONFIG_WATCH_QUEUE is not set
 # CONFIG_CROSS_MEMORY_ATTACH is not set
@@ -937,7 +938,9 @@ CONFIG_DUMMY_CONSOLE_ROWS=25
 # end of Console display driver support
 # end of Graphics support
 
-# CONFIG_SOUND is not set
+CONFIG_SOUND=y
+CONFIG_SND=y
+CONFIG_SND_VIRTIO=y
 
 #
 # HID support

configs/linux.config

@@ -306,6 +306,53 @@ void aclint_sswi_write(hart_t *hart,
                        uint8_t width,
                        uint32_t value);
 
+/* VirtIO-Sound */
+
+#if SEMU_HAS(VIRTIOSND)
+#define IRQ_VSND 5
+#define IRQ_VSND_BIT (1 << IRQ_VSND)
+
+typedef struct {
+    uint32_t QueueNum;
+    uint32_t QueueDesc;
+    uint32_t QueueAvail;
+    uint32_t QueueUsed;
+    uint16_t last_avail;
+    bool ready;
+} virtio_snd_queue_t;
+
+typedef struct {
+    /* feature negotiation */
+    uint32_t DeviceFeaturesSel;
+    uint32_t DriverFeatures;
+    uint32_t DriverFeaturesSel;
+    /* queue config */
+    uint32_t QueueSel;
+    virtio_snd_queue_t queues[4];
+    /* status */
+    uint32_t Status;
+    uint32_t InterruptStatus;
+    /* supplied by environment */
+    uint32_t *ram;
+    /* implementation-specific */
+    void *priv;
+} virtio_snd_state_t;
+
+void virtio_snd_read(hart_t *core,
+                     virtio_snd_state_t *vsnd,
+                     uint32_t addr,
+                     uint8_t width,
+                     uint32_t *value);
+
+void virtio_snd_write(hart_t *core,
+                      virtio_snd_state_t *vsnd,
+                      uint32_t addr,
+                      uint8_t width,
+                      uint32_t value);
+
+bool virtio_snd_init(virtio_snd_state_t *vsnd);
+#endif /* SEMU_HAS(VIRTIOSND) */
+
 /* memory mapping */
 typedef struct {
     bool stopped;
@@ -326,4 +373,7 @@ typedef struct {
     mtimer_state_t mtimer;
     mswi_state_t mswi;
     sswi_state_t sswi;
+#if SEMU_HAS(VIRTIOSND)
+    virtio_snd_state_t vsnd;
+#endif
 } emu_state_t;

device.h

@@ -12,5 +12,10 @@
 #define SEMU_FEATURE_VIRTIONET 1
 #endif
 
+/* virtio-snd */
+#ifndef SEMU_FEATURE_VIRTIOSND
+#define SEMU_FEATURE_VIRTIOSND 1
+#endif
+
 /* Feature test macro */
 #define SEMU_HAS(x) SEMU_FEATURE_##x

feature.h

@@ -100,6 +100,18 @@ static void emu_update_swi_interrupt(hart_t *hart)
     aclint_sswi_update_interrupts(hart, &data->sswi);
 }
 
+#if SEMU_HAS(VIRTIOSND)
+static void emu_update_vsnd_interrupts(vm_t *vm)
+{
+    emu_state_t *data = PRIV(vm->hart[0]);
+    if (data->vsnd.InterruptStatus)
+        data->plic.active |= IRQ_VSND_BIT;
+    else
+        data->plic.active &= ~IRQ_VSND_BIT;
+    plic_update_interrupts(vm, &data->plic);
+}
+#endif
+
 static void mem_load(hart_t *hart,
                      uint32_t addr,
                      uint8_t width,
@@ -155,6 +167,13 @@ static void mem_load(hart_t *hart,
             emu_update_vrng_interrupts(hart->vm);
             return;
 #endif
+
+#if SEMU_HAS(VIRTIOSND)
+        case 0x47: /* virtio-snd */
+            virtio_snd_read(hart, &data->vsnd, addr & 0xFFFFF, width, value);
+            emu_update_vsnd_interrupts(hart->vm);
+            return;
+#endif
         }
     }
     vm_set_exception(hart, RV_EXC_LOAD_FAULT, hart->exc_val);
@@ -209,12 +228,20 @@ 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
+
+#if SEMU_HAS(VIRTIOSND)
+        case 0x47: /* virtio-snd */
+            virtio_snd_write(hart, &data->vsnd, addr & 0xFFFFF, width, value);
+            emu_update_vsnd_interrupts(hart->vm);
+            return;
+#endif
         }
     }
     vm_set_exception(hart, RV_EXC_STORE_FAULT, hart->exc_val);
@@ -651,6 +678,11 @@ static int semu_start(int argc, char **argv)
     emu.mtimer.mtimecmp = calloc(vm.n_hart, sizeof(uint64_t));
     emu.mswi.msip = calloc(vm.n_hart, sizeof(uint32_t));
     emu.sswi.ssip = calloc(vm.n_hart, sizeof(uint32_t));
+#if SEMU_HAS(VIRTIOSND)
+    if (!virtio_snd_init(&(emu.vsnd)))
+        fprintf(stderr, "No virtio-snd functioned\n");
+    emu.vsnd.ram = emu.ram;
+#endif
 
     /* Emulate */
     uint32_t peripheral_update_ctr = 0;
@@ -676,6 +708,11 @@ static int semu_start(int argc, char **argv)
                 if (emu.vblk.InterruptStatus)
                     emu_update_vblk_interrupts(&vm);
 #endif
+
+#if SEMU_HAS(VIRTIOSND)
+                if (emu.vsnd.InterruptStatus)
+                    emu_update_vsnd_interrupts(&vm);
+#endif
             }
 
             emu_update_timer_interrupt(vm.hart[i]);

main.c

@@ -71,5 +71,13 @@
             interrupts = <4>;
         };
 #endif
+
+#if SEMU_FEATURE_VIRTIOSND
+        snd0: virtio@4700000 {
+            compatible = "virtio,mmio";
+            reg = <0x4700000 0x200>;
+            interrupts = <5>;
+        };
+#endif
     };
 };

minimal.dts

@@ -0,0 +1,17 @@
+# Create a mininal ALSA program
+define create-alsa-prog
+echo '\
+#include <alsa/asoundlib.h>\n\
+int main(){\n\
+    snd_pcm_t *pcm;\n\
+    snd_pcm_open(&pcm, "default", SND_PCM_STREAM_PLAYBACK, 0);\n\
+    snd_pcm_close(pcm);\n\
+    return 0;\n\
+}\n'
+endef
+
+# Check ALSA installation
+define check-alsa
+$(shell $(call create-alsa-prog) | $(CC) -x c -lasound -o /dev/null > /dev/null 2> /dev/null - 
+	&& echo $$?)
+endef