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Date:   Mon, 13 Sep 2021 13:01:25 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Lai Jiangshan" <jiangshanlai@gmail.com>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Cc:     "Lai Jiangshan" <laijs@linux.alibaba.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Dave Hansen" <dave.hansen@linux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Dave Jiang" <dave.jiang@intel.com>,
        "Ben Widawsky" <ben.widawsky@intel.com>,
        "Williams, Dan J" <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
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On Thu, Sep 2, 2021, at 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.
> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>

>  
>  #ifdef CONFIG_X86_64
> +
> +/*
> + * Reload gs selector with exception handling
> + * selector:  new selector
> + *
> + * Is noinstr as it shouldn't be instrumented.
> + */
> +noinstr void native_load_gs_index(unsigned int selector)
> +{
> +	unsigned long flags;
> +
> +	local_irq_save(flags);

This patch would be a bit less alarming if you moved the swapgs into asm.  Also, this needs a comment explaining why skipping the swapgs back to kernel gs in the exception path is correct.

> +	native_swapgs();
> +	asm volatile(
> +		".global asm_load_gs_index_gs_change \n"
> +		"asm_load_gs_index_gs_change: \n"
> +		"1: movl %0, %%gs \n"
> +		"   swapgs \n"
> +		"2: \n"
> +		_ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_clear_gs)
> +		:: "r" (selector) : "memory");
> +	alternative("", "mfence", X86_BUG_SWAPGS_FENCE);
> +	local_irq_restore(flags);
> +}
> +