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Date: Wed, 10 Jan 2018 17:22:45 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Brian Gerst <brgerst@...il.com>
Cc: Andi Kleen <andi@...stfloor.org>,
Thomas Gleixner <tglx@...utronix.de>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Woodhouse <dwmw@...zon.co.uk>,
Paul Turner <pjt@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Tom Lendacky <thomas.lendacky@....com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
Dave Hansen <dave.hansen@...el.com>,
Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call
> On Jan 10, 2018, at 5:01 PM, Brian Gerst <brgerst@...il.com> wrote:
>
>> On Wed, Jan 10, 2018 at 7:55 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>>
>>>> On Jan 10, 2018, at 4:16 PM, Andi Kleen <andi@...stfloor.org> wrote:
>>>>
>>>>> On Tue, Jan 09, 2018 at 09:46:16PM -0500, Brian Gerst wrote:
>>>>> On Tue, Jan 9, 2018 at 8:03 PM, Andi Kleen <andi@...stfloor.org> wrote:
>>>>> From: Andi Kleen <ak@...ux.intel.com>
>>>>>
>>>>> Remove the partial stack frame in the 64bit syscall fast path.
>>>>> In the next patch we want to clear the extra registers, which requires
>>>>> to always save all registers. So remove the partial stack frame
>>>>> in the syscall fast path and always save everything.
>>>>>
>>>>> This actually simplifies the code because the ptregs stubs
>>>>> are not needed anymore.
>>>>>
>>>>> arch/x86/entry/entry_64.S | 57 ++++-----------------------------------------------------
>>>>> arch/x86/entry/syscall_64.c | 2 +-
>>>>>
>>>>> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
>>>>> ---
>>>>> arch/x86/entry/entry_64.S | 57 ++++-----------------------------------------
>>>>> arch/x86/entry/syscall_64.c | 2 +-
>>>>> 2 files changed, 5 insertions(+), 54 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>>>> index 58dbf7a12a05..bbdfbdd817d6 100644
>>>>> --- a/arch/x86/entry/entry_64.S
>>>>> +++ b/arch/x86/entry/entry_64.S
>>>>> @@ -234,7 +234,9 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
>>>>> pushq %r9 /* pt_regs->r9 */
>>>>> pushq %r10 /* pt_regs->r10 */
>>>>> pushq %r11 /* pt_regs->r11 */
>>>>> - sub $(6*8), %rsp /* pt_regs->bp, bx, r12-15 not saved */
>>>>> + sub $(6*8), %rsp
>>>>> + SAVE_EXTRA_REGS
>>>>> +
>>>>
>>>> Continue using pushes here
>>>>
>>>>> UNWIND_HINT_REGS extra=0
>>>>>
>>>>> /*
>>>>> @@ -262,11 +264,6 @@ entry_SYSCALL_64_fastpath:
>>>>> ja 1f /* return -ENOSYS (already in pt_regs->ax) */
>>>>> movq %r10, %rcx
>>>>>
>>>>> - /*
>>>>> - * This call instruction is handled specially in stub_ptregs_64.
>>>>> - * It might end up jumping to the slow path. If it jumps, RAX
>>>>> - * and all argument registers are clobbered.
>>>>> - */
>>>>> #ifdef CONFIG_RETPOLINE
>>>>> movq sys_call_table(, %rax, 8), %rax
>>>>> call __x86_indirect_thunk_rax
>>>>> @@ -293,9 +290,7 @@ entry_SYSCALL_64_fastpath:
>>>>> TRACE_IRQS_ON /* user mode is traced as IRQs on */
>>>>> movq RIP(%rsp), %rcx
>>>>> movq EFLAGS(%rsp), %r11
>>>>> - addq $6*8, %rsp /* skip extra regs -- they were preserved */
>>>>> - UNWIND_HINT_EMPTY
>>>>> - jmp .Lpop_c_regs_except_rcx_r11_and_sysret
>>>>> + jmp syscall_return_via_sysret
>>>>>
>>>>> 1:
>>>>> /*
>>>>> @@ -305,14 +300,12 @@ entry_SYSCALL_64_fastpath:
>>>>> */
>>>>> TRACE_IRQS_ON
>>>>> ENABLE_INTERRUPTS(CLBR_ANY)
>>>>> - SAVE_EXTRA_REGS
>>>>> movq %rsp, %rdi
>>>>> call syscall_return_slowpath /* returns with IRQs disabled */
>>>>> jmp return_from_SYSCALL_64
>>>>>
>>>>> entry_SYSCALL64_slow_path:
>>>>> /* IRQs are off. */
>>>>> - SAVE_EXTRA_REGS
>>>>> movq %rsp, %rdi
>>>>> call do_syscall_64 /* returns with IRQs disabled */
>>>>>
>>>>> @@ -389,7 +382,6 @@ syscall_return_via_sysret:
>>>>> /* rcx and r11 are already restored (see code above) */
>>>>> UNWIND_HINT_EMPTY
>>>>> POP_EXTRA_REGS
>>>>> -.Lpop_c_regs_except_rcx_r11_and_sysret:
>>>>> popq %rsi /* skip r11 */
>>>>> popq %r10
>>>>> popq %r9
>>>>> @@ -420,47 +412,6 @@ syscall_return_via_sysret:
>>>>> USERGS_SYSRET64
>>>>> END(entry_SYSCALL_64)
>>>>>
>>>>> -ENTRY(stub_ptregs_64)
>>>>> - /*
>>>>> - * Syscalls marked as needing ptregs land here.
>>>>> - * If we are on the fast path, we need to save the extra regs,
>>>>> - * which we achieve by trying again on the slow path. If we are on
>>>>> - * the slow path, the extra regs are already saved.
>>>>> - *
>>>>> - * RAX stores a pointer to the C function implementing the syscall.
>>>>> - * IRQs are on.
>>>>> - */
>>>>> - cmpq $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
>>>>> - jne 1f
>>>>> -
>>>>> - /*
>>>>> - * Called from fast path -- disable IRQs again, pop return address
>>>>> - * and jump to slow path
>>>>> - */
>>>>> - DISABLE_INTERRUPTS(CLBR_ANY)
>>>>> - TRACE_IRQS_OFF
>>>>> - popq %rax
>>>>> - UNWIND_HINT_REGS extra=0
>>>>> - jmp entry_SYSCALL64_slow_path
>>>>> -
>>>>> -1:
>>>>> - JMP_NOSPEC %rax /* Called from C */
>>>>> -END(stub_ptregs_64)
>>>>> -
>>>>> -.macro ptregs_stub func
>>>>> -ENTRY(ptregs_\func)
>>>>> - UNWIND_HINT_FUNC
>>>>> - leaq \func(%rip), %rax
>>>>> - jmp stub_ptregs_64
>>>>> -END(ptregs_\func)
>>>>> -.endm
>>>>> -
>>>>> -/* Instantiate ptregs_stub for each ptregs-using syscall */
>>>>> -#define __SYSCALL_64_QUAL_(sym)
>>>>> -#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_stub sym
>>>>> -#define __SYSCALL_64(nr, sym, qual) __SYSCALL_64_QUAL_##qual(sym)
>>>>> -#include <asm/syscalls_64.h>
>>>>> -
>>>>
>>>> You can't just blindly remove this. We need to make sure that
>>>> syscalls that modify registers take the slow path exit, because they
>>>> may change the registers to be incompatible with SYSRET.
>>>
>>> That's a good point. I checked the ptregs calls:
>>>
>>> iopl: should be fine, we will be restoring the correct IOPL through
>>> SYSRET
>>>
>>> clone/fork: fine too, the original return is fine and ret_from_fork
>>> takes care of the child
>>>
>>> execve et.al.: we will be leaking r11(rflags), rcx(orig return) into
>>> the new process. but that seems acceptable.
>>>
>>> rt_sigreturn: that's the only one who has problems. I added a new
>>> TIF_FULL_RESTORE to force it into the slow path.
>>>
>>
>> So your series removes the old declarative annotation and then will add a new TI flag to make it work again?
>>
>> This whole thing seems to be at the wrong end of the cost benefit curve.
>
> We already check TIF flags after the syscall on the fast path. Adding
> another bit to the mask costs nothing.
>
What I mean is: this whole series is almost certainly a performance regression, it has no off switch, and is doesn't obviously solve any problem. It' didn't qualify as a so. And no one has benchmarked it. I think we should seriously consider just not applying it.
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