Re: [PATCH 13/43] x86/entry/64: Use a percpu trampoline stack for IDT entries

[Thomas Gleixner]

On Fri, 24 Nov 2017, Ingo Molnar wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxx>
> @@ -563,6 +563,13 @@ END(irq_entries_start)
>  /* 0(%rsp): ~(interrupt number) */
>  	.macro interrupt func
>  	cld
> +
> +	testb	$3, CS-ORIG_RAX(%rsp)
> +	jz	1f
> +	SWAPGS
> +	call	switch_to_thread_stack
> +1:

Yes, that's what I thought it should look like.

>  	ALLOC_PT_GPREGS_ON_STACK
>  	SAVE_C_REGS
>  	SAVE_EXTRA_REGS
> @@ -572,12 +579,8 @@ END(irq_entries_start)
>  	jz	1f

If you change that to 2f and adjust the label down there it gets even
simpler to read. I know it works, but I still find it disturbing.

>  	/*
> -	 * IRQ from user mode.  Switch to kernel gsbase and inform context
> -	 * tracking that we're in kernel mode.
> -	 */
> -	SWAPGS
> -
> -	/*
> +	 * IRQ from user mode.
> +	 *
>  	 * We need to tell lockdep that IRQs are off.  We can't do this until
>  	 * we fix gsbase, and we should do it before enter_from_user_mode
>  	 * (which can take locks).  Since TRACE_IRQS_OFF idempotent,
> @@ -831,6 +834,32 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
>   */
>  #define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8)
>  
> +/*
> + * Switch to the thread stack.  This is called with the IRET frame and
> + * orig_ax on the stack.  (That is, RDI..R12 are not on the stack and
> + * space has not been allocated for them.)
> + */
> +ENTRY(switch_to_thread_stack)
> +	UNWIND_HINT_FUNC
> +
> +	pushq	%rdi
> +	movq	%rsp, %rdi
> +	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> +	UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI
> +
> +	pushq	7*8(%rdi)		/* regs->ss */
> +	pushq	6*8(%rdi)		/* regs->rsp */
> +	pushq	5*8(%rdi)		/* regs->eflags */
> +	pushq	4*8(%rdi)		/* regs->cs */
> +	pushq	3*8(%rdi)		/* regs->ip */
> +	pushq	2*8(%rdi)		/* regs->orig_ax */
> +	pushq	8(%rdi)			/* return address */
> +	UNWIND_HINT_FUNC
> +
> +	movq	(%rdi), %rdi
> +	ret
> +END(switch_to_thread_stack)

Much nicer.

>  .macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
>  ENTRY(\sym)
>  	UNWIND_HINT_IRET_REGS offset=\has_error_code*8
> @@ -848,11 +877,12 @@ ENTRY(\sym)
>  
>  	ALLOC_PT_GPREGS_ON_STACK
>  
> -	.if \paranoid
> -	.if \paranoid == 1
> +	.if \paranoid < 2
>  	testb	$3, CS(%rsp)			/* If coming from userspace, switch stacks */
> -	jnz	1f
> +	jnz	.Lfrom_usermode_switch_stack_\@
>  	.endif
> +
> +	.if \paranoid
>  	call	paranoid_entry
>  	.else
>  	call	error_entry
> @@ -894,20 +924,15 @@ ENTRY(\sym)
>  	jmp	error_exit
>  	.endif
>  
> -	.if \paranoid == 1
> +	.if \paranoid < 2
>  	/*
> -	 * Paranoid entry from userspace.  Switch stacks and treat it
> +	 * Entry from userspace.  Switch stacks and treat it
>  	 * as a normal entry.  This means that paranoid handlers
>  	 * run in real process context if user_mode(regs).
>  	 */
> -1:
> +.Lfrom_usermode_switch_stack_\@:
>  	call	error_entry
>  
> -
> -	movq	%rsp, %rdi			/* pt_regs pointer */
> -	call	sync_regs
> -	movq	%rax, %rsp			/* switch stack */
> -
>  	movq	%rsp, %rdi			/* pt_regs pointer */
>  
>  	.if \has_error_code
> @@ -1170,6 +1195,14 @@ ENTRY(error_entry)
>  	SWAPGS
>  
>  .Lerror_entry_from_usermode_after_swapgs:
> +	/* Put us onto the real thread stack. */
> +	popq	%r12				/* save return addr in %12 */
> +	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
> +	call	sync_regs
> +	movq	%rax, %rsp			/* switch stack */
> +	ENCODE_FRAME_POINTER
> +	pushq	%r12
> +

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>