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([2804:7f0:bcc0:8f02:a679:333c:d017:a155]) by smtp.gmail.com with ESMTPSA id ada2fe7eead31-4e6444964casm9865070137.13.2025.06.04.03.54.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 03:54:42 -0700 (PDT) Message-ID: Date: Wed, 4 Jun 2025 07:54:38 -0300 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] target/riscv/kvm: implement SBI debug console (DBCN) calls To: =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= , 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> <37e6fecd-948a-4840-9189-918d44c0be72@linaro.org> Content-Language: en-US From: Daniel Henrique Barboza In-Reply-To: <37e6fecd-948a-4840-9189-918d44c0be72@linaro.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::930; envelope-from=dbarboza@ventanamicro.com; helo=mail-ua1-x930.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 On 6/4/25 6:38 AM, Philippe Mathieu-Daudé wrote: > (+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. I can handle this change, don't worry about it. I'll send it out today. > >> >> 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)". > This make sense to me. It also goes in line with what Daniel mentioned in his reply. I'll make the change with this semantic in mind. Thanks, Daniel > 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. >> >