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From: Lai Jiangshan <laijs@linux.alibaba.com>
To: "H. Peter Anvin" <hpa@zytor.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org
Cc: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Jiang <dave.jiang@intel.com>,
	Ben Widawsky <ben.widawsky@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Arvind Sankar <nivedita@alum.mit.edu>
Subject: Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code
Date: Wed, 8 Sep 2021 15:12:06 +0800	[thread overview]
Message-ID: <f76296d5-8d95-cf3d-b800-3f6b2e2d21fb@linux.alibaba.com> (raw)
In-Reply-To: <638f3b2b-aff9-72e5-3a5d-fff5ef6b88fc@zytor.com>



On 2021/9/8 13:00, H. Peter Anvin wrote:
> On 9/7/21 9:42 PM, H. Peter Anvin wrote:
>>
>>
>> On 9/7/21 6:38 PM, H. Peter Anvin wrote:
>>> On 9/2/21 3:50 AM, Lai Jiangshan wrote:
>>>> From: Lai Jiangshan <laijs@linux.alibaba.com>
>>>>
>>>> There is no constrain/limition to force native_load_gs_index() to be in
>>>> ASM code.
>>>
>>> Hi,
>>>
>>> First of all, let me say I really like your patchset, and I will try to
>>> review it in detail ASAP (most of the initial read pass looks very sane
>>> to me.

Hello

Thank you for your review.

>>>
>>> However, I would like to object in part this specific patch. It adds a
>>> fair bit of extra code to the exception path, and adds jumps between
>>> files which makes the code much harder to read.

I tried putting all code into a single C function.
But I didn't know how to use a C-label in _ASM_EXTABLE and then I split it.

Your code is much better.

Thanks
Lai


>>>
>>> You end up doing one swapgs in assembly and one in C, which would seem
>>> to be a very good indication that really isn't an improvement.
>>>
>>> Note that this entire sequence is scheduled to be obsoleted by a single
>>> atomic hardware instruction, LKGS, which will replace ALL of
>>> native_load_gs_index(); it will no longer be necessary even to disable
>>> interrupts as there is no non-atomic state. In that sense, doing this as
>>> an out-of-line C function (with some inline assembly) is great, because
>>> it makes it far easier to use LKGS as an alternative; the only (small)
>>> disadvantage is that it ends up clobbering a lot of registers
>>> unnecessarily (in assembly it can be implemented clobbering only two
>>> registers; one if one uses pushf/popf to save the interrupt flag.)
>>>
>>
>> OK, here is a version which actually compiles:
>>
> 
> ... slightly shorter and minimally better compiled code ...
> 
> noinstr void native_load_gs_index(unsigned int selector)
> {
>      unsigned long flags;
> 
>      local_irq_save(flags);
>      native_swapgs();
> do_mov_gs:
>      asm_volatile_goto("1: mov %[seg],%%gs\n"
>                "2:\n"
>                _ASM_EXTABLE(1b,%l[bad_seg])
>                : : [seg] "r" (selector) : : bad_seg);
>      alternative("", "mfence", X86_BUG_SWAPGS_FENCE);
>      native_swapgs();
>      local_irq_restore(flags);
>      return;
> 
> bad_seg:
>      /* The exception dispatch will have restored kernel GS */
>      native_swapgs();
>      alternative_input("", "mov %[seg],%%gs",
>                X86_BUG_NULL_SEG, [seg] "r" (__USER_DS));
>      selector = 0;
>      goto do_mov_gs;
> }

  reply	other threads:[~2021-09-08  7:12 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-31 17:50 [PATCH 00/24] x86/entry/64: Convert a bunch of ASM entry code into " Lai Jiangshan
2021-08-31 17:50 ` [PATCH 01/24] x86/traps: Remove stack-protector from traps.c Lai Jiangshan
2021-08-31 17:50 ` [PATCH 02/24] x86/traps: Move arch/x86/kernel/traps.c to arch/x86/entry/ Lai Jiangshan
2021-09-02  8:09   ` Joerg Roedel
2021-09-02  9:21     ` Lai Jiangshan
2021-09-02 10:50       ` Peter Zijlstra
2021-09-02 11:54         ` Lai Jiangshan
2021-09-02 12:05           ` Peter Zijlstra
2021-09-02 13:34             ` Peter Zijlstra
2021-09-02 17:05               ` Nick Desaulniers
2021-09-02 17:19                 ` Miguel Ojeda
2021-09-02 17:23                   ` Nick Desaulniers
2021-09-03  7:36                 ` Martin Liška
2021-09-07 21:12                   ` Nick Desaulniers
2021-09-08  7:33                     ` Martin Liška
2021-08-31 17:50 ` [PATCH 03/24] x86/traps: Move declaration of native_irq_return_iret up Lai Jiangshan
2021-08-31 17:50 ` [PATCH 04/24] x86/entry: Expose the address of .Lgs_change to traps.c Lai Jiangshan
2021-09-02  9:14   ` Peter Zijlstra
2021-09-02  9:20     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 05/24] x86/entry: Introduce __entry_text for entry code written in C Lai Jiangshan
2021-08-31 19:34   ` Peter Zijlstra
2021-09-01  0:23     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 06/24] x86/entry: Move PTI_USER_* to arch/x86/include/asm/processor-flags.h Lai Jiangshan
2021-08-31 17:50 ` [PATCH 07/24] x86: Mark __native_read_cr3() & native_write_cr3() as __always_inline Lai Jiangshan
2021-08-31 17:50 ` [PATCH 08/24] x86/traps: Add C verion of SWITCH_TO_KERNEL_CR3 as switch_to_kernel_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 09/24] x86/traps: Add fence_swapgs_{user,kernel}_entry() Lai Jiangshan
2021-09-02  9:25   ` Peter Zijlstra
2021-08-31 17:50 ` [PATCH 10/24] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 11/24] x86/entry: Replace the most of asm code of error_entry to C code Lai Jiangshan
2021-09-02 10:16   ` Peter Zijlstra
2021-09-02 12:08     ` Lai Jiangshan
2021-08-31 17:50 ` [PATCH 12/24] x86/traps: Reconstruct pt_regs on task stack directly in fixup_bad_iret() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 13/24] x86/traps: Mark sync_regs() and fixup_bad_iret() as static __always_inline Lai Jiangshan
2021-08-31 17:50 ` [PATCH 14/24] x86/entry: Make paranoid_exit() callable Lai Jiangshan
2021-08-31 17:50 ` [PATCH 15/24] x86/entry: Call paranoid_exit() in asm_exc_nmi() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 16/24] x86/entry: Use skip_rdi instead of save_ret for PUSH_AND_CLEAR_REGS Lai Jiangshan
2021-08-31 17:50 ` [PATCH 17/24] x86/entry: Introduce struct ist_regs Lai Jiangshan
2021-09-10  0:18   ` Lai Jiangshan
2021-09-10  0:36     ` Lai Jiangshan
2021-09-10  4:31     ` H. Peter Anvin
2021-09-10  7:13       ` Lai Jiangshan
2021-09-10  7:14         ` H. Peter Anvin
2021-09-10  4:50     ` H. Peter Anvin
2021-09-10  4:51       ` H. Peter Anvin
2021-08-31 17:50 ` [PATCH 18/24] x86/entry: Add the C version ist_switch_to_kernel_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 19/24] x86/entry: Add the C version ist_restore_cr3() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 20/24] x86/entry: Add the C version get_percpu_base() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 21/24] x86/entry: Add the C version ist_switch_to_kernel_gsbase() Lai Jiangshan
2021-08-31 17:50 ` [PATCH 22/24] x86/entry: Implement and use do_paranoid_entry() and paranoid_exit() Lai Jiangshan
2021-09-02 10:33   ` Peter Zijlstra
2021-09-02 10:42     ` Lai Jiangshan
2021-09-02 12:02       ` Peter Zijlstra
2021-09-02 11:58     ` Lai Jiangshan
2021-09-02 12:29       ` Joerg Roedel
2021-08-31 17:50 ` [PATCH 23/24] x86/entry: Remove the unused ASM macros Lai Jiangshan
2021-08-31 17:50 ` [PATCH 24/24] x86/syscall/64: Move the checking for sysret to C code Lai Jiangshan
2021-09-10  7:20   ` Nikolay Borisov
2021-09-10  7:30     ` Lai Jiangshan
2021-08-31 20:44 ` [PATCH 00/24] x86/entry/64: Convert a bunch of ASM entry code into " Peter Zijlstra
2021-09-02  6:28   ` Lai Jiangshan
2021-09-02  7:44     ` Peter Zijlstra
2021-09-02 10:50 ` [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in " Lai Jiangshan
2021-09-08  1:38   ` H. Peter Anvin
2021-09-08  4:42     ` H. Peter Anvin
2021-09-08  5:00       ` H. Peter Anvin
2021-09-08  7:12         ` Lai Jiangshan [this message]
2021-09-09 23:16           ` H. Peter Anvin
2021-09-13 20:01   ` Andy Lutomirski
2021-09-14  2:04     ` Lai Jiangshan
2021-09-14  8:14       ` Peter Zijlstra
2021-09-14  8:17         ` Borislav Petkov
2021-09-14  8:40         ` Lai Jiangshan

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