Re: [PATCH 07/12] x86/entry/64: Always run ptregs-using syscalls on the slow path

[Andy Lutomirski]

On Mon, Dec 7, 2015 at 4:54 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
> On Mon, Dec 7, 2015 at 7:50 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> On Mon, Dec 7, 2015 at 4:51 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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
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