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SubjectRe: [PATCH 13/43] x86/entry/64: Use a percpu trampoline stack for IDT entries
On Fri, 24 Nov 2017, Ingo Molnar wrote:
> From: Andy Lutomirski <luto@kernel.org>
> @@ -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@linutronix.de>

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