* perf: FEX's in-root path lookups go through the fast path instead of proot
Android has no openat2, so libblsession walks RESOLVE_IN_ROOT lookups for
FEX one component at a time. Every fstatat, readlinkat and openat in that
walk was a raw syscall that stopped in proot. When libblfastpath is loaded,
the walk now calls its fstatat, readlinkat and openat, which answer without
proot and fall back to the raw syscall themselves. Without it the walk uses
the raw syscalls as before.
One lookup drops from 250-940 us to 26-86 us. A Graveyard Keeper launch
makes 37.5k proot stops instead of 54.0k and reaches its first scene
0.31 s sooner.
* test: count fstatat64 where the preload asks for it
* perf: FEX's own file opens go through the fast path instead of proot
FEX opens host files for the game with syscall(SYS_openat), so every one
stopped in proot even when the fast path could answer it. libblsession's
syscall() now sends SYS_openat to the fast path's openat as well, the same
way the in-root walk does, and makes the raw syscall when the fast path is
not loaded.
A Graveyard Keeper launch makes 28.0k proot stops instead of 37.5k; FEX's
openat stops go from 6,739 to none.
* fix: the fast path's start-up reads don't come back through syscall()
libblsession's syscall() now sends SYS_openat to the fast path. A process
named FEX starts the fast path by reading /proc/self/cmdline with
syscall(SYS_openat), which came straight back into the fast path and
waited on its own pthread_once. Its start-up reads now call libc's
syscall directly.
MaxsTechReview
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