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Subject: Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        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>
References: <20210831175025.27570-1-jiangshanlai@gmail.com>
 <20210902105052.2842-1-jiangshanlai@gmail.com>
 <9fdb04b1-dbb8-069d-f5ef-d4e8c0f2a83e@zytor.com>
Message-ID: <f84c2b3c-a880-502f-4f51-4624b3800a05@zytor.com>
Date:   Tue, 7 Sep 2021 21:42:43 -0700
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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.
> 
> 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.
> 
> 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:

(It makes me wonder if it would be useful to have an _ASM_EXTABLE_GOTO() 
macro to highlight the use of an asm goto C label.)

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:
	selector = 0;

	/* The exception dispatch will have restored kernel GS */
	native_swapgs();

	if (static_cpu_has_bug(X86_BUG_NULL_SEG))
		asm volatile("mov %[seg],%%gs"
			     : : [seg] "r" (__USER_DS));
	goto do_mov_gs;
}