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SubjectRe: [RFC] x86: save_args out of line
On Sun, Nov 16, 2008 at 03:29:01PM +0100, Alexander van Heukelum wrote:
> From: Alexander van Heukelum <heukelum@sleipnir.lusi.uni-sb.de>
>
> The macro "interrupt" in entry_64.S generates a lot of code. This
> patch moves most of its contents into an external function. It
> saves anywhere between 500 and 2500 bytes of text depending on the
> configuration.
>
> The function returns with an altered stackpointer.
>
> Dwarf2-annotations are most probably wrong or missing.
>
> There is a comment in the original code about saving rbp twice, but
> I don't understand what the code tries to do. First of all, the
> second copy of rbp is written below the stack.
This is not for the stack, but to keep the registers as the pt_regs struct
expects them. We'll be casting this structure later, and not doing that
can lead to bogus code, whenever we cast regs->bp.

> Second, if the current
> stack is already the irqstack, this second copy is overwritten. Third,
> as far as I can tell, ebp should not be saved in pt_regs at all at
> this stage as it is 'preserved' due to the C calling conventions. So
> I left this second copy out and everything seems to work fine. If it
> wouldn't, all exceptions and NMIs would be dangerous anyhow, as far
> as I can see.
Try testing with a kernel with CONFIG_FRAME_POINTER turned on.

>
> Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
> Cc: Glauber Costa <gcosta@redhat.com>
>
> ---
> arch/x86/kernel/entry_64.S | 123 ++++++++++++++++++++++++++------------------
> 1 files changed, 73 insertions(+), 50 deletions(-)
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index a3a7fb2..eb57c23 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -247,6 +247,78 @@ ENTRY(native_usergs_sysret64)
> CFI_REL_OFFSET rsp,RSP
> /*CFI_REL_OFFSET ss,SS*/
> .endm
> +
> +/*
> + * initial frame state for interrupts and exceptions
> + */
> + .macro _frame ref
> + CFI_STARTPROC simple
> + CFI_SIGNAL_FRAME
> + CFI_DEF_CFA rsp,SS+8-\ref
> + /*CFI_REL_OFFSET ss,SS-\ref*/
> + CFI_REL_OFFSET rsp,RSP-\ref
> + /*CFI_REL_OFFSET rflags,EFLAGS-\ref*/
> + /*CFI_REL_OFFSET cs,CS-\ref*/
> + CFI_REL_OFFSET rip,RIP-\ref
> + .endm
> +
> +/* initial frame state for interrupts (and exceptions without error code) */
> +#define INTR_FRAME _frame RIP
> +/* initial frame state for exceptions with error code (and interrupts with
> + vector already pushed) */
> +#define XCPT_FRAME _frame ORIG_RAX
> +
> +/* save_args changes the stack */
> +ENTRY(save_args)
> + XCPT_FRAME
> + cld
> + subq $9*8, %rsp
> + CFI_ADJUST_CFA_OFFSET 9*8
> + push 9*8(%rsp) /* copy return address */
> + CFI_ADJUST_CFA_OFFSET 8
> + movq %rdi, 8*8+16(%rsp)
> + CFI_REL_OFFSET rdi, 8*8+16
> + movq %rsi, 7*8+16(%rsp)
> + CFI_REL_OFFSET rsi, 7*8+16
> + movq %rdx, 6*8+16(%rsp)
> + CFI_REL_OFFSET rdx, 6*8+16
> + movq %rcx, 5*8+16(%rsp)
> + CFI_REL_OFFSET rcx, 5*8+16
> + movq %rax, 4*8+16(%rsp)
> + CFI_REL_OFFSET rax, 4*8+16
> + movq %r8, 3*8+16(%rsp)
> + CFI_REL_OFFSET r8, 3*8+16
> + movq %r9, 2*8+16(%rsp)
> + CFI_REL_OFFSET r9, 2*8+16
> + movq %r10, 1*8+16(%rsp)
> + CFI_REL_OFFSET r10, 1*8+16
> + movq %r11, 0*8+16(%rsp)
> + CFI_REL_OFFSET r11, 0*8+16
> + leaq -ARGOFFSET+16(%rsp),%rdi /* arg1 for handler */
> + movq %rbp, 8(%rsp) /* push %rbp */
> + leaq 8(%rsp), %rbp /* mov %rsp, %ebp */
> + testl $3, CS(%rdi)
> + je 1f
> + SWAPGS
> + /*
> + * irqcount is used to check if a CPU is already on an interrupt stack
> + * or not. While this is essentially redundant with preempt_count it is
> + * a little cheaper to use a separate counter in the PDA (short of
> + * moving irq_enter into assembly, which would be too much work)
> + */
> +1: incl %gs:pda_irqcount
> + jne 2f
> + pop %rax /* move return address... */
> + mov %gs:pda_irqstackptr,%rsp
> + push %rax /* ... to the new stack */
> + /*
> + * We entered an interrupt context - irqs are off:
> + */
> +2: TRACE_IRQS_OFF
> + ret
> + CFI_ENDPROC
> +END(save_args)
> +
> /*
> * A newly forked process directly context switches into this.
> */
> @@ -615,26 +687,6 @@ ENTRY(stub_rt_sigreturn)
> END(stub_rt_sigreturn)
>
> /*
> - * initial frame state for interrupts and exceptions
> - */
> - .macro _frame ref
> - CFI_STARTPROC simple
> - CFI_SIGNAL_FRAME
> - CFI_DEF_CFA rsp,SS+8-\ref
> - /*CFI_REL_OFFSET ss,SS-\ref*/
> - CFI_REL_OFFSET rsp,RSP-\ref
> - /*CFI_REL_OFFSET rflags,EFLAGS-\ref*/
> - /*CFI_REL_OFFSET cs,CS-\ref*/
> - CFI_REL_OFFSET rip,RIP-\ref
> - .endm
> -
> -/* initial frame state for interrupts (and exceptions without error code) */
> -#define INTR_FRAME _frame RIP
> -/* initial frame state for exceptions with error code (and interrupts with
> - vector already pushed) */
> -#define XCPT_FRAME _frame ORIG_RAX
> -
> -/*
> * Interrupt entry/exit.
> *
> * Interrupt entry points save only callee clobbered registers in fast path.
> @@ -644,36 +696,7 @@ END(stub_rt_sigreturn)
>
> /* 0(%rsp): interrupt number */
> .macro interrupt func
> - cld
> - SAVE_ARGS
> - leaq -ARGOFFSET(%rsp),%rdi # arg1 for handler
> - pushq %rbp
> - /*
> - * Save rbp twice: One is for marking the stack frame, as usual, and the
> - * other, to fill pt_regs properly. This is because bx comes right
> - * before the last saved register in that structure, and not bp. If the
> - * base pointer were in the place bx is today, this would not be needed.
> - */
> - movq %rbp, -8(%rsp)
> - CFI_ADJUST_CFA_OFFSET 8
> - CFI_REL_OFFSET rbp, 0
> - movq %rsp,%rbp
> - CFI_DEF_CFA_REGISTER rbp
> - testl $3,CS(%rdi)
> - je 1f
> - SWAPGS
> - /* irqcount is used to check if a CPU is already on an interrupt
> - stack or not. While this is essentially redundant with preempt_count
> - it is a little cheaper to use a separate counter in the PDA
> - (short of moving irq_enter into assembly, which would be too
> - much work) */
> -1: incl %gs:pda_irqcount
> - cmoveq %gs:pda_irqstackptr,%rsp
> - push %rbp # backlink for old unwinder
> - /*
> - * We entered an interrupt context - irqs are off:
> - */
> - TRACE_IRQS_OFF
> + call save_args
> call \func
> .endm
>
> --
> 1.5.4.3
>


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