Re: [PATCH] x86/entry/64: Remove duplicate syscall table for fast path

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Dec 9, 2015 at 5:02 AM, Brian Gerst <brgerst@gmail.com> wrote:
> Instead of using a duplicate syscall table for the fast path, create stubs for
> the syscalls that need pt_regs that save the extra registers if a flag for the
> slow path is not set.
>
> Signed-off-by: Brian Gerst <brgerst@gmail.com>
> To: Andy Lutomirski <luto@amacapital.net>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: the arch/x86 maintainers <x86@kernel.org>
> Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Frédéric Weisbecker <fweisbec@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
>
> Applies on top of Andy's syscall cleanup series.

A couple questions:

> @@ -306,15 +306,37 @@ END(entry_SYSCALL_64)
>
>  ENTRY(stub_ptregs_64)
>         /*
> -        * Syscalls marked as needing ptregs that go through the fast path
> -        * land here.  We transfer to the slow path.
> +        * Syscalls marked as needing ptregs land here.
> +        * If we are on the fast path, we need to save the extra regs.
> +        * If we are on the slow path, the extra regs are already saved.
>          */
> -       DISABLE_INTERRUPTS(CLBR_NONE)
> -       TRACE_IRQS_OFF
> -       addq    $8, %rsp
> -       jmp     entry_SYSCALL64_slow_path
> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %r10
> +       testl   $TS_SLOWPATH, ASM_THREAD_INFO(TI_status, %r10, 0)
> +       jnz     1f

OK (but see below), but why not do:

addq $8, %rsp
jmp entry_SYSCALL64_slow_path

here instead of the stack munging below?

> +       subq    $SIZEOF_PTREGS, %r10
> +       SAVE_EXTRA_REGS base=r10
> +       movq    %r10, %rbx
> +       call    *%rax
> +       movq    %rbx, %r10
> +       RESTORE_EXTRA_REGS base=r10
> +       ret
> +1:
> +       jmp     *%rax
>  END(stub_ptregs_64)

Also, can we not get away with keying off rip or rsp instead of
ti->status?  That should be faster and less magical IMO.

--Andy