On Mon, Dec 7, 2015 at 4:54 PM, Brian Gerst <brge...@gmail.com> wrote: > On Mon, Dec 7, 2015 at 7:50 PM, Brian Gerst <brge...@gmail.com> wrote: >> On Mon, Dec 7, 2015 at 4:51 PM, Andy Lutomirski <l...@kernel.org> wrote: >>> 64-bit syscalls currently have an optimization in which they are >>> called with partial pt_regs. A small handful require full pt_regs. >>> >>> In the 32-bit and compat cases, I cleaned this up by forcing full >>> pt_regs for all syscalls. The performance hit doesn't really matter. >>> >>> I want to clean up the 64-bit case as well, but I don't want to hurt >>> fast path performance. To do that, I want to force the syscalls >>> that use pt_regs onto the slow path. This will enable us to make >>> slow path syscalls be real ABI-compliant C functions. >>> >>> Use the new syscall entry qualification machinery for this. >>> stub_clone is now stub_clone/ptregs. >>> >>> The next patch will eliminate the stubs, and we'll just have >>> sys_clone/ptregs. > > [Resend after gmail web interface fail] > > I've got an idea on how to do this without the duplicate syscall table. > > ptregs_foo: > leaq sys_foo(%rip), %rax > jmp stub_ptregs_64 > > stub_ptregs_64: > testl $TS_EXTRAREGS, <current->ti_status> > jnz 1f > SAVE_EXTRA_REGS > call *%rax > RESTORE_EXTRA_REGS > ret > 1: > call *%rax > ret > > This makes sure that the extra regs don't get saved a second time if > coming in from the slow path, but preserves the fast path if not > tracing.
I think there's value in having the entries in the table be genuine C ABI-compliant function pointers. In your example, it only barely works -- you can call them from C only if you have TS_EXTRAREGS set appropriately -- -otherwise you crash and burn. That will break the rest of the series. We could adjust it a bit and check whether we're in C land (by checking rsp for ts) and jump into the slow path if we aren't, but I'm not sure this is a huge win. It does save some rodata space by avoiding duplicating the table. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
