From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7D24EC5B552 for ; Wed, 4 Jun 2025 09:39:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uMkZw-0006Jo-4X; Wed, 04 Jun 2025 05:38:36 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uMkZt-0006IJ-5t for qemu-devel@nongnu.org; Wed, 04 Jun 2025 05:38:33 -0400 Received: from mail-wr1-x42a.google.com ([2a00:1450:4864:20::42a]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1uMkZq-0008KE-6R for qemu-devel@nongnu.org; Wed, 04 Jun 2025 05:38:32 -0400 Received: by mail-wr1-x42a.google.com with SMTP id ffacd0b85a97d-3a4f72cba73so550772f8f.1 for ; Wed, 04 Jun 2025 02:38:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; t=1749029908; x=1749634708; darn=nongnu.org; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :from:to:cc:subject:date:message-id:reply-to; bh=kJNbYih2BhiYcF2yhC06sYNzsVwoT8BqId9NFTFNV4o=; b=OvPLyCRCO1i5SRftFC7WqIOfJTn0WtSCRY8tg3ZwqICSEUzETGNtKz7mS/VJsye9In yqxiJYMilEuMSvuideUOj7+4S5Lxsz0ud4n1EoNykoBYwsLJt4AdArghPT+ufqWSypu+ 8PXwCR8DLR3oYbOic9Hi8JCim/xX4r7ev8ui6w+iAWsOlTn54VhHiSQcTlc/Kz0FWyHc tREpHVYd9ZdwQEye3/CyjLEbPr1rO/cLm76+iVlYJZyePSGPOG9wWjlWRkZ+fwFHQYjw heoMDhWVvqnWFKUvOPaaW/gfZsX7F/v/hc18GWRH2AOe3rymfobX41RrNA7Vrmkr39xQ m68A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1749029908; x=1749634708; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=kJNbYih2BhiYcF2yhC06sYNzsVwoT8BqId9NFTFNV4o=; b=HVm0j7dA75sM06XatPS6REvWIIJDyQLzMsYZ5YYGfwUNlaIfXJq1GzGndX6r5Oh1hH uHPEh0f+5GGkrZpUhb8nQ+wW/vuCxSs+tIQDQgMZo9V5b28IZX4TPzpQe0gxZit/UM/s KpWjJfUpK4r7AdHmOK04utXzXFA6796e0NVBB3zhb+JRPXsZiZv94TvYOHbcWnz5U6t7 6TukqCjW7q8YQ2xQGfgoEW+AgA+Km4f4LlI7dYp4WiBYNXiAECJWJLfBZutyAEDY2M8f M+m9VCJhCgpPeoBa0e/Dbwe5c8/vg5Gw3uiiJ3ZHqyive6r2X3xBEcup3711OBlP8QJu YUeA== X-Forwarded-Encrypted: i=1; AJvYcCUYKjhSTMdge7djd86jA6FkcqoY5PmW+GIu7/C5HLkkke7IB/ZtPk6LnwxsdjcbUttf01SPXM9NEoYA@nongnu.org X-Gm-Message-State: AOJu0YxzZg85bOVQk/3ivjDI3Jgo/38PFSCj1qmvHNpCAvIohM6PZtwo j9oat20nG/lg3aTaBCsXOLiqSt6EbETsOBInBGzxjMXWniENYpAaEj3Q4+WoKC+yXbM= X-Gm-Gg: ASbGncsIvgQty3CWUGmEmVxvMn76UrIw1tKNkf7LQdDi8v/DgCn/QZEHTYqk2Xp/jmN 2hO4RoMdePl18139NT7qqsLM0wcyMVLUiMcvXqCuzUqvizdH6ObkOBsg0Td5JaxJxBqhtPATdaD D4yYf+rDjmgD9+4FMxkRDbKTXbiWK0rhEhrAzqw7yIhGtzPBO/2Pa7Ur9uVKSn7p91lA7U9Vv6D dPUI48dEKbfxAKc6IoxS5xFv6xhf7w5hKBzbI466Qztus1ElYg0/ifjKs1TFGiKJ0IZf1ZZSk3I olF9rBrg+YM7dWVuUjnD/F1S2Qu6K8Gu3iAcghHUXcMhxMSwrJs+jAdtyCQIHPMDKWy60ZVo6rj O74Oye+34zFooblaV9l4= X-Google-Smtp-Source: AGHT+IEZ7xsJ2k6VLih5klOWD2HSL2NrnoE8RCo4oxmA7Yf71rfFYWSl4Q64YKsgcmUNa2XL6CLLXg== X-Received: by 2002:a5d:64ed:0:b0:3a4:eee4:cdec with SMTP id ffacd0b85a97d-3a514168e0cmr4683015f8f.6.1749029908252; Wed, 04 Jun 2025 02:38:28 -0700 (PDT) Received: from [192.168.69.138] (88-187-86-199.subs.proxad.net. [88.187.86.199]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4efe73f3csm20764531f8f.42.2025.06.04.02.38.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 02:38:27 -0700 (PDT) Message-ID: <37e6fecd-948a-4840-9189-918d44c0be72@linaro.org> Date: Wed, 4 Jun 2025 11:38:26 +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, Peter Maydell , =?UTF-8?Q?Marc-Andr=C3=A9_Lureau?= , Paolo Bonzini References: <20240425155012.581366-1-dbarboza@ventanamicro.com> <102e2e67-4c4d-4912-a892-20f5136f241a@ventanamicro.com> <47327da2-be3c-4aaa-ab3d-36e8d0c25185@linaro.org> <0eb5bcf9-bc2e-46c6-9f54-1514039557e0@ventanamicro.com> Content-Language: en-US From: =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= In-Reply-To: <0eb5bcf9-bc2e-46c6-9f54-1514039557e0@ventanamicro.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::42a; envelope-from=philmd@linaro.org; helo=mail-wr1-x42a.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=unavailable 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 (+Marc-André and Paolo who I forgot to Cc first) On 4/6/25 11:17, Daniel Henrique Barboza wrote: > > > On 6/4/25 4:32 AM, Philippe Mathieu-Daudé wrote: >> 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. > > I think I got the gist of it, thanks. > > For this particular console call the spec says: > > "This is a blocking SBI call and it will only return after writing the > specified > byte to the debug console. It will also return, with SBI_ERR_FAILED, if > there are > I/O errors." > > > So I think it pairs well with the blocking version qemu_chr_fe_write_all() > instead. I can do this change and get out of your way in changing the > callers > of qemu_chr_fe_write(). I appreciate if you post the patch (this is a 1 line change, but what matters here is the justification you just provided), but if you are busy I can do it, I have enough information to write the commit desc. > > But I still have questions, hehe. This blocking version has the following > doc: > > "(...) Unlike @qemu_chr_fe_write, this function will block if the back end > cannot consume all of the data attempted to be written. This function is > thread-safe. > > Returns: the number of bytes consumed (0 if no associated Chardev) > or -1 on error." > > Do we have plans to change this API like we're doing with the non-blocking > version? Because being a blocking call that promises "block until all bytes > are written", and I have len  > 0, I don't expect a ret = 0 to be interpret > as "no bytes were written".  I am ok with ret = 0 being 'no associated > chardev' > and not handling it as an error (for now at least) but I would like to > confirm > that qemu_chr_fe_write_all() will not interpret ret = 0 as a zero byte > write. > In other words, if for some reason other than "no chardev present" we > ended up > with zero bytes written I would like a ret < 0 return. Correct. Clarifying this method is in my TODO. What about this example: - frontend wants to block to write 8 bytes - backend writes 5 bytes, hits an unrecoverable link error Should the backend support be responsible to re-establish link and retry (when possible) to complete? If we return -1 for error, could the frontend try to reconnect and write the 8 bytes, ending with the first 5 bytes being transmitted twice? Meanwhile I'm thinking to document as: "Returns @len on success, or -1 on error (some data might has been written)". Simpler API could be: "Returns 0 on success, otherwise -errno" but we'd need to rework all the callers. > > > Thanks, > > Daniel > > > >> >>> 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. >