Date: Wed, 31 Jan 2024 12:27:58 -0400 From: Mitchell Horne <mhorne@freebsd.org> To: Lin Lee <leelin2602@gmail.com>, freebsd-riscv@freebsd.org Subject: Re: A little bit wondering about how a syscall works Message-ID: <b4e54e75-fcfd-475c-b0f7-6be7a6cf5e84@freebsd.org> In-Reply-To: <f2213278-0807-4a36-ab90-6d2a9555fc8c@Spark> References: <9204b5c5-63bc-4c29-af19-1f1bb85f74e8@Spark> <8f66f3ed-2a49-4ebc-89eb-66c53e6d22bb@Spark> <9bfbdbe2-ff30-4c2f-abf3-2763aa433107@freebsd.org> <f2213278-0807-4a36-ab90-6d2a9555fc8c@Spark>
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On 1/31/24 01:03, Lin Lee wrote: > Hello Mitchell, > > Thank you for your kindly responding. > > Now I have still a question, when does the function > cpu_fetch_syscall_args be called? > > As the previous letter mentions, I traced the code and entered the > elf_machdep.c. > > I have no idea if there are something to do between elf_machdep.c and > system calll. > The short answer is yes, it is related. In syscallenter() we have: error = (p->p_sysent->sv_fetch_syscall_args)(td); And as you saw, the sv_fetch_syscall_args hook is set to cpu_fetch_syscall_args() for elf64_freebsd_sysvec. Similarly, there is an sv_set_syscall_retval hook, called by syscallret() when we are done executing the system call. Each process 'p' has a corresponding sysentvec (p_sysent). On the riscv architecture there is currently only one registered systentvec, elf64_freebsd_sysvec, because we can only execute 64-bit FreeBSD ELF binaries on this platform. By contrast, on amd64 there are several registered sysentvecs. This allows it to execute, for example, 32-bit FreeBSD ELF binaries, or 64-bit Linux ELF binaries. The sysentvec enables different handling for these different types of executables, e.g. the system call table is different for Linux processes (.sv_table = linux_sysent). You will see also that Linux processes have a different function for sv_fetch_syscall_args, take a look in sys/amd64/linux/linux_sysvec.c. Mitchell > If not, when(how) the cpu_fetch_syscall_args is called? > > Thank you very much. > > Best Regards, > Lin Lee > On Jan 31, 2024 at 1:17 AM +0800, Mitchell Horne <mhorne@freebsd.org>, > wrote: >> >> Mitchell
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