diff options
Diffstat (limited to 'hurd')
| -rw-r--r-- | hurd/io_path.mdwn | 71 |
1 files changed, 62 insertions, 9 deletions
diff --git a/hurd/io_path.mdwn b/hurd/io_path.mdwn index 96e6aa58..b55dc20f 100644 --- a/hurd/io_path.mdwn +++ b/hurd/io_path.mdwn @@ -10,18 +10,71 @@ is included in the section entitled # read - * [[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_read`. +[[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_read`. - * That calls `glibc/hurd/fd-read.c:_hurd_fd_read()`. +A buffer (and its size) to store the to-be-read data in is supplied by the +caller of `read`. - * That calls `__io_read`, which is an [[RPC]], i.e., that actually results - into the [[translator/ext2fs]] server calling - `hurd/libdiskfs/io-read.c:diskfs_S_io_read`. +> `__libc_read` calls `glibc/hurd/fd-read.c:_hurd_fd_read`. - * That calls `_diskfs_rdwr_internal`, which calls - `hurd/libpager/pager-memcpy.c:pager_memcpy`, which usually basically just - tell the kernel to virtually project the memory object corresponding to the - file in the caller process's memory. No read is actually done. +>> `_hurd_fd_read` calls `__io_read`, which is an [[RPC]]: +>> `hurd/hurd/io.defs:io_read`. + +>>> Enter user-side RPC stub `glibc.obj/hurd/RPC_io_read.c:__io_read`. Process +>>> stuff, switch to kernel, etc. + +(For example) [[translator/hello]] server, [[libtrivfs]]-based. Enter +server-side RPC stub `hurd.obj/libtrivfs/ioServer.c:_Xio_read`. Process stuff, +call `hurd/trans/hello.c:trivfs_S_io_read`. + +A 2048 byte buffer is provided. + +> `trivfs_S_io_read`. Depending on the internatl state, either a new memory +> region is set-up (and returned as out-of-line data), or the desired amount of +> data is returned in-line. + +Back in `_Xio_read`. + +If the 2048 byte buffer is not decided to be used (out-of-line case or bigger +than 2048 bytes case; server decides to instead provide a new memory region), +the [[`dealloc`|microkernel/mach/mig/dealloc]] flag is being set, which causes +Mach to unmap that memory region from the server's address space, i.e., doing a +memory *move* from the server to the client. + +Leave server-side RPC stub `_Xio_read`. + +>>> Return from kernel, continue client-side RPC stub `io_read`. Have to copy +>>> data. Three cases: out-of-line data (pass pointer to memory area); +>>> returned more data than fits into the originally supplied buffer (allocate +>>> new buffer, copy all data into it, pass pointer of new buffer); otherwise +>>> copy as much data as is available into the originally supplied buffer. +>>> I.e., in all cases *all* data which was provided by the server is made +>>> available to the caller. + +>> Back in `_hurd_fd_read`. If a new buffer has been allocated previously, or +>> the out-of-line mechanism has been used, the returned data now has to be +>> copied into the originally supplied buffer. If the server returned more +>> data than requested, this is a [[protocol_violation|EGRATUITOUS]]. + +> Back in `__libc_read`. + + +--- + +Samuel: + +(For example) [[translator/ext2fs]] server, enter server-side RPC stub +`hurd.obj/libdiskfs/ioServer.c:_Xio_read`. Process stuff, call +`hurd/libdiskfs/io-read.c:diskfs_S_io_read`. + +A 2048 byte buffer is provided. + +> `diskfs_S_io_read` calls `_diskfs_rdwr_internal`. + +>> That calls `hurd/libpager/pager-memcpy.c:pager_memcpy`, which usually +>> basically just tells the kernel to virtually project the memory object +>> corresponding to the file in the caller process's memory. No read is +>> actually done. * Then, when the process actually reads the data, the kernel gets the user page fault (`gnumach/i386/i386/trap.c:user_trap`), which calls `vm_fault`, |