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[88.187.86.199]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4f0097539sm21212778f8f.63.2025.06.04.00.32.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 00:32:22 -0700 (PDT) Message-ID: <47327da2-be3c-4aaa-ab3d-36e8d0c25185@linaro.org> Date: Wed, 4 Jun 2025 09:32:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] target/riscv/kvm: implement SBI debug console (DBCN) calls To: Daniel Henrique Barboza , qemu-devel@nongnu.org Cc: qemu-riscv@nongnu.org, alistair.francis@wdc.com, bmeng@tinylab.org, liwei1518@gmail.com, zhiwei_liu@linux.alibaba.com, palmer@rivosinc.com, Peter Maydell References: <20240425155012.581366-1-dbarboza@ventanamicro.com> <102e2e67-4c4d-4912-a892-20f5136f241a@ventanamicro.com> Content-Language: en-US From: =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= In-Reply-To: <102e2e67-4c4d-4912-a892-20f5136f241a@ventanamicro.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::431; envelope-from=philmd@linaro.org; helo=mail-wr1-x431.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 3/6/25 20:04, Daniel Henrique Barboza wrote: > > > On 6/3/25 10:19 AM, Philippe Mathieu-Daudé wrote: >> Hi Daniel, >> >> (now merged as commit a6b53378f537) >> >> On 25/4/24 17:50, Daniel Henrique Barboza wrote: >>> SBI defines a Debug Console extension "DBCN" that will, in time, replace >>> the legacy console putchar and getchar SBI extensions. >>> >>> The appeal of the DBCN extension is that it allows multiple bytes to be >>> read/written in the SBI console in a single SBI call. >>> >>> As far as KVM goes, the DBCN calls are forwarded by an in-kernel KVM >>> module to userspace. But this will only happens if the KVM module >>> actually supports this SBI extension and we activate it. >>> >>> We'll check for DBCN support during init time, checking if get-reg-list >>> is advertising KVM_RISCV_SBI_EXT_DBCN. In that case, we'll enable it via >>> kvm_set_one_reg() during kvm_arch_init_vcpu(). >>> >>> Finally, change kvm_riscv_handle_sbi() to handle the incoming calls for >>> SBI_EXT_DBCN, reading and writing as required. >>> >>> A simple KVM guest with 'earlycon=sbi', running in an emulated RISC-V >>> host, takes around 20 seconds to boot without using DBCN. With this >>> patch we're taking around 14 seconds to boot due to the speed-up in the >>> terminal output.  There's no change in boot time if the guest isn't >>> using earlycon. >>> >>> Signed-off-by: Daniel Henrique Barboza >>> --- >>>   target/riscv/kvm/kvm-cpu.c         | 111 +++++++++++++++++++++++++++++ >>>   target/riscv/sbi_ecall_interface.h |  17 +++++ >>>   2 files changed, 128 insertions(+) >> >> >>> +static void kvm_riscv_handle_sbi_dbcn(CPUState *cs, struct kvm_run >>> *run) >>> +{ >>> +    g_autofree uint8_t *buf = NULL; >>> +    RISCVCPU *cpu = RISCV_CPU(cs); >>> +    target_ulong num_bytes; >>> +    uint64_t addr; >>> +    unsigned char ch; >>> +    int ret; >>> + >>> +    switch (run->riscv_sbi.function_id) { >>> +    case SBI_EXT_DBCN_CONSOLE_READ: >>> +    case SBI_EXT_DBCN_CONSOLE_WRITE: >>> +        num_bytes = run->riscv_sbi.args[0]; >>> + >>> +        if (num_bytes == 0) { >>> +            run->riscv_sbi.ret[0] = SBI_SUCCESS; >>> +            run->riscv_sbi.ret[1] = 0; >>> +            break; >>> +        } >>> + >>> +        addr = run->riscv_sbi.args[1]; >>> + >>> +        /* >>> +         * Handle the case where a 32 bit CPU is running in a >>> +         * 64 bit addressing env. >>> +         */ >>> +        if (riscv_cpu_mxl(&cpu->env) == MXL_RV32) { >>> +            addr |= (uint64_t)run->riscv_sbi.args[2] << 32; >>> +        } >>> + >>> +        buf = g_malloc0(num_bytes); >>> + >>> +        if (run->riscv_sbi.function_id == SBI_EXT_DBCN_CONSOLE_READ) { >>> +            ret = qemu_chr_fe_read_all(serial_hd(0)->be, buf, >>> num_bytes); >>> +            if (ret < 0) { >>> +                error_report("SBI_EXT_DBCN_CONSOLE_READ: error when " >>> +                             "reading chardev"); >>> +                exit(1); >>> +            } >>> + >>> +            cpu_physical_memory_write(addr, buf, ret); >>> +        } else { >>> +            cpu_physical_memory_read(addr, buf, num_bytes); >>> + >>> +            ret = qemu_chr_fe_write_all(serial_hd(0)->be, buf, >>> num_bytes); >>> +            if (ret < 0) { >>> +                error_report("SBI_EXT_DBCN_CONSOLE_WRITE: error when " >>> +                             "writing chardev"); >>> +                exit(1); >>> +            } >>> +        } >>> + >>> +        run->riscv_sbi.ret[0] = SBI_SUCCESS; >>> +        run->riscv_sbi.ret[1] = ret; >>> +        break; >>> +    case SBI_EXT_DBCN_CONSOLE_WRITE_BYTE: >>> +        ch = run->riscv_sbi.args[0]; >>> +        ret = qemu_chr_fe_write(serial_hd(0)->be, &ch, sizeof(ch)); >>> + >>> +        if (ret < 0) { >>> +            error_report("SBI_EXT_DBCN_CONSOLE_WRITE_BYTE: error when " >>> +                         "writing chardev"); >>> +            exit(1); >>> +        } >> >> We are ignoring partial writes (non-blocking call returning 0 byte >> written), is that expected? If so, is it OK to add a comment we can >> safely discard not-yet-written DBCN_CONSOLE_WRITE_BYTE? > > Not sure what you meant. IIUC qemu_chr_fe_write() returns the number > of bytes consumed, 0 if no chardev is found, and -1 on error. I'm trying to address an issue Peter reported with qemu_chr_fe_write(): https://lore.kernel.org/qemu-devel/CAFEAcA_kEndvNtw4EHySXWwQPoGs029yAzZGGBcV=zGHaj7KUQ@mail.gmail.com/ Basically upon introduction in commit cd18720a294 in 2013 ("char: introduce a blocking version of qemu_chr_fe_write") the API contract was "Returns: the number of bytes consumed" which could be 0, so some frontends return 0 for "wrote no bytes". Later in 2016 in commit fa394ed6257 ("char: make some qemu_chr_fe skip if no driver") the API documentation was changed: - * Returns: the number of bytes consumed + * Returns: the number of bytes consumed (0 if no assicated CharDriver) After this commit, some frontends started to handle '<=0' as error, while 0 is not an error. > Are you > saying that we should do a loop when there's no chardev found (ret = 0) > and wait a certain time until there's one available? > > > In fact, seeing how SBI_EXT_DBCN_CONSOLE_WRITE is written, I wonder if > we could use qemu_chr_fe_write_all() in this case too. This is certainly simpler. Regards, Phil.