Re: [PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code

[Andy Lutomirski]

On Thu, Sep 2, 2021, at 3:50 AM, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> There is no constrain/limition to force native_load_gs_index() to be in
> ASM code.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

>  
>  #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);
> +}
> +