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SubjectRe: [PATCH v2 13/18] x86/asm/64: Use a percpu trampoline stack for IDT entries
On Tue, 21 Nov 2017, Andy Lutomirski wrote:
> The asm isn't exactly beautiful,

Delightful euphemism :)

> but I think that fully refactoring
> it can wait.

> @@ -560,6 +560,14 @@ END(irq_entries_start)
> .macro interrupt func
> cld
> ALLOC_PT_GPREGS_ON_STACK
> +
> + testb $3, CS(%rsp)
> + jz 1f
> + SWAPGS
> + call switch_to_thread_stack
> + SWAPGS

I'm surely missing something subtle, but the register saving does really
not care on which GS it is. This swapgs orgy looks odd.

> +1:
> +
> SAVE_C_REGS
> SAVE_EXTRA_REGS
> ENCODE_FRAME_POINTER
> @@ -827,6 +835,33 @@ 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 in pt_regs and the rest of pt_regs allocated, but with all GPRs
> + * in the CPU registers.

That took several attempts to grok why you left ALLOC_PT_GPRES_ON_STACK in
place in the interrupts macro above.

In theory it would be sufficient to push %rdi on the entry stack and
operate from there, but it spares only the 'addq %rsp'. Not worth the
trouble of dealing with different register offsets.

A comment to that effect would be useful if you look at that 3 month from
now.

> + */
> +ENTRY(switch_to_thread_stack)
> + UNWIND_HINT_IRET_REGS offset=17*8
> +
> + movq %rdi, RDI+8(%rsp)
> + movq %rsp, %rdi
> + movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> + UNWIND_HINT_IRET_REGS offset=17*8 base=%rdi
> +
> + pushq SS+8(%rdi) /* regs->ss */
> + pushq RSP+8(%rdi) /* regs->rsp */
> + pushq EFLAGS+8(%rdi) /* regs->eflags */
> + pushq CS+8(%rdi) /* regs->cs */
> + pushq RIP+8(%rdi) /* regs->ip */
> + pushq ORIG_RAX+8(%rdi) /* regs->orig_ax */
> + ALLOC_PT_GPREGS_ON_STACK /* allocate the rest of regs */
> + pushq (%rdi) /* return address */
> +
> + movq RDI+8(%rdi), %rdi
> + UNWIND_HINT_IRET_REGS offset=17*8
> + ret
> +END(switch_to_thread_stack)

Thanks,

tglx

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