riscv: use -bios option for QEMU 4.1

And remove prebuilt OpenSBI for virt, since it's included in QEMU.
Ref: https://qemu.weilnetz.de/doc/qemu-doc.html#Deprecated-features B.1.15

README.md

@@ -21,7 +21,7 @@ Supported architectures and boards:
 
 * [Rust](https://www.rust-lang.org) toolchain
 * Cargo tools: [cargo-xbuild](https://github.com/rust-osdev/cargo-xbuild), [cargo-binutils](https://github.com/rust-embedded/cargo-binutils)
-* [QEMU](https://www.qemu.org) >= 3.1.0
+* [QEMU](https://www.qemu.org) >= 4.1.0
 * [musl-based GCC toolchains](https://musl.cc/) (only for building [user programs](https://github.com/rcore-os/rcore-user))
 
 Setup on Linux or macOS:

kernel/Makefile

@@ -118,7 +118,7 @@ else ifeq ($(arch), riscv32)
 qemu_opts += \
 	-machine virt \
 	-serial mon:stdio \
-	-kernel ../tools/opensbi/virt_rv32.elf \
+	-bios default \
 	-device loader,addr=0x80400000,file=$(kernel_img) \
 	-drive file=$(USER_QCOW2),format=qcow2,id=sfs \
 	-device virtio-blk-device,drive=sfs \
@@ -133,13 +133,13 @@ ifeq ($(board), u540)
 qemu_opts += \
 	-machine virt \
 	-serial mon:stdio \
-	-kernel ../tools/opensbi/fu540.elf \
+	-bios ../tools/opensbi/fu540.elf \
 	-device loader,addr=0x80200000,file=$(kernel_img)
 else
 qemu_opts += \
 	-machine virt \
 	-serial mon:stdio \
-	-kernel ../tools/opensbi/virt_rv64.elf \
+	-bios default \
 	-device loader,addr=0x80200000,file=$(kernel_img) \
 	-drive file=$(USER_QCOW2),format=qcow2,id=sfs \
 	-device virtio-blk-device,drive=sfs \

tools/opensbi/README.md

@@ -1,9 +1,6 @@
 # OpenSBI
 
-These are binary release of OpenSBI v0.4.
-
-- virt_rv32.elf: opensbi-0.4-rv32-bin/platform/qemu/virt/firmware/fw_jump.elf
-- virt_rv64.elf: opensbi-0.4-rv64-bin/platform/qemu/virt/firmware/fw_jump.elf
+These are binary release of OpenSBI v0.3.
 
 NOTE: The [official v0.3 release](https://github.com/riscv/opensbi/releases/tag/v0.3) has bug on serial interrupt. Also, Rocket-Chip based CPUs (including SiFive Unleashed) seem to have unintended behavior on