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Date: Thu, 4 Sep 2014 14:57:49 -0400 (EDT)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Daniel Thompson <daniel.thompson@...aro.org>
cc: Russell King <linux@....linux.org.uk>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
patches@...aro.org, linaro-kernel@...ts.linaro.org,
John Stultz <john.stultz@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sumit Semwal <sumit.semwal@...aro.org>,
Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v1 1/6] arm: fiq: Replace default FIQ handler
On Thu, 4 Sep 2014, Daniel Thompson wrote:
> This patch introduces a new default FIQ handler that is structured in a
> similar way to the existing ARM exception handler and result in the FIQ
> being handled by C code running on the SVC stack (despite this code run
> in the FIQ handler is subject to severe limitations with respect to
> locking making normal interaction with the kernel impossible).
>
> This default handler allows concepts that on x86 would be handled using
> NMIs to be realized on ARM.
>
> Credit:
>
> This patch is a near complete re-write of a patch originally
> provided by Anton Vorontsov. Today only a couple of small fragments
> survive, however without Anton's work to build from this patch would
> not exist.
>
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: Nicolas Pitre <nico@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> ---
> arch/arm/kernel/entry-armv.S | 110 +++++++++++++++++++++++++++++++++++++++----
> arch/arm/kernel/setup.c | 8 +++-
> arch/arm/kernel/traps.c | 26 ++++++++--
> 3 files changed, 130 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 36276cd..03dc0e0 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -146,7 +146,7 @@ ENDPROC(__und_invalid)
> #define SPFIX(code...)
> #endif
>
> - .macro svc_entry, stack_hole=0
> + .macro svc_entry, stack_hole=0, call_trace=1
> UNWIND(.fnstart )
> UNWIND(.save {r0 - pc} )
> sub sp, sp, #(S_FRAME_SIZE + \stack_hole - 4)
> @@ -183,10 +183,35 @@ ENDPROC(__und_invalid)
> stmia r7, {r2 - r6}
>
> #ifdef CONFIG_TRACE_IRQFLAGS
> + .if \call_trace
> bl trace_hardirqs_off
> + .endif
> #endif
> .endm
>
> +@
> +@ svc_exit_via_fiq - similar to svc_exit but switches to FIQ mode before exit
> +@
> +@ This macro acts in a similar manner to svc_exit but switches to FIQ
> +@ mode to restore the final part of the register state.
> +@
> +@ We cannot use the normal svc_exit procedure because that would
> +@ clobber spsr_svc (FIQ could be delivered during the first few instructions
> +@ of vector_swi meaning its contents have not been saved anywhere).
> +@
Wouldn't it be better for this macro to live in entry-header.S alongside
the others? Also you should probably create a Thumb2 version.
> + .macro svc_exit_via_fiq, rpsr
> +
> + mov r0, sp
> + ldmib r0, {r1 - r14} @ abort is deadly from here onward (it will
> + @ clobber state restored below)
> + msr cpsr_c, #FIQ_MODE | PSR_I_BIT | PSR_F_BIT
> + add r8, r0, #S_PC
> + ldr r9, [r0, #S_PSR]
> + msr spsr_cxsf, r9
> + ldr r0, [r0, #S_R0]
> + ldmia r8, {pc}^
> + .endm
> +
> .align 5
> __dabt_svc:
> svc_entry
> @@ -295,6 +320,15 @@ __pabt_svc:
> ENDPROC(__pabt_svc)
>
> .align 5
> +__fiq_svc:
> + svc_entry 0, 0
> + mov r0, sp @ struct pt_regs *regs
> + bl handle_fiq_as_nmi
> + svc_exit_via_fiq r5
> + UNWIND(.fnend )
> +ENDPROC(__fiq_svc)
> +
> + .align 5
> .LCcralign:
> .word cr_alignment
> #ifdef MULTI_DABORT
> @@ -305,6 +339,38 @@ ENDPROC(__pabt_svc)
> .word fp_enter
>
> /*
> + * Abort mode handlers
> + */
> +
> +@
> +@ Taking a FIQ in abort mode is similar to taking a FIQ in SVC mode
> +@ and reuses the same macros. However in abort mode we must also
> +@ save/restore lr_abt and spsr_abt to make nested aborts safe.
> +@
> + .align 5
> +__fiq_abt:
> + svc_entry 0, 0
> +
> + msr cpsr_c, #ABT_MODE | PSR_I_BIT | PSR_F_BIT
> + mov r0, lr @ Save lr_abt
> + mrs r1, spsr @ Save spsr_abt, abort is now safe
> + msr cpsr_c, #SVC_MODE | PSR_I_BIT | PSR_F_BIT
> + push {r0 - r1}
> +
> + sub r0, sp, #8 @ struct pt_regs *regs
> + bl handle_fiq_as_nmi
> +
> + pop {r0 - r1}
> + msr cpsr_c, #ABT_MODE | PSR_I_BIT | PSR_F_BIT
> + mov lr, r0 @ Restore lr_abt, abort is unsafe
> + msr spsr_cxsf, r1 @ Restore spsr_abt
> + msr cpsr_c, #SVC_MODE | PSR_I_BIT | PSR_F_BIT
> +
> + svc_exit_via_fiq r5
> + UNWIND(.fnend )
> +ENDPROC(__fiq_svc)
> +
> +/*
> * User mode handlers
> *
> * EABI note: sp_svc is always 64-bit aligned here, so should S_FRAME_SIZE
> @@ -683,6 +749,18 @@ ENTRY(ret_from_exception)
> ENDPROC(__pabt_usr)
> ENDPROC(ret_from_exception)
>
> + .align 5
> +__fiq_usr:
> + usr_entry
> + kuser_cmpxchg_check
> + mov r0, sp @ struct pt_regs *regs
> + bl handle_fiq_as_nmi
> + get_thread_info tsk
> + mov why, #0
> + b ret_to_user_from_irq
> + UNWIND(.fnend )
> +ENDPROC(__fiq_usr)
> +
> /*
> * Register switch for ARMv3 and ARMv4 processors
> * r0 = previous task_struct, r1 = previous thread_info, r2 = next thread_info
> @@ -1118,17 +1196,29 @@ vector_addrexcptn:
> b vector_addrexcptn
>
> /*=============================================================================
> - * Undefined FIQs
> + * FIQ "NMI" handler
> *-----------------------------------------------------------------------------
> - * Enter in FIQ mode, spsr = ANY CPSR, lr = ANY PC
> - * MUST PRESERVE SVC SPSR, but need to switch to SVC mode to show our msg.
> - * Basically to switch modes, we *HAVE* to clobber one register... brain
> - * damage alert! I don't think that we can execute any code in here in any
> - * other mode than FIQ... Ok you can switch to another mode, but you can't
> - * get out of that mode without clobbering one register.
> + * Handle a FIQ using the SVC stack allowing FIQ act like NMI on x86
> + * systems.
> */
> -vector_fiq:
> - subs pc, lr, #4
> + vector_stub fiq, FIQ_MODE, 4
> +
> + .long __fiq_usr @ 0 (USR_26 / USR_32)
> + .long __fiq_svc @ 1 (FIQ_26 / FIQ_32)
> + .long __fiq_svc @ 2 (IRQ_26 / IRQ_32)
> + .long __fiq_svc @ 3 (SVC_26 / SVC_32)
> + .long __fiq_svc @ 4
> + .long __fiq_svc @ 5
> + .long __fiq_svc @ 6
> + .long __fiq_abt @ 7
> + .long __fiq_svc @ 8
> + .long __fiq_svc @ 9
> + .long __fiq_svc @ a
> + .long __fiq_svc @ b
> + .long __fiq_svc @ c
> + .long __fiq_svc @ d
> + .long __fiq_svc @ e
> + .long __fiq_svc @ f
>
> .globl vector_fiq_offset
> .equ vector_fiq_offset, vector_fiq
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 84db893d..c031063 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -133,6 +133,7 @@ struct stack {
> u32 irq[3];
> u32 abt[3];
> u32 und[3];
> + u32 fiq[3];
> } ____cacheline_aligned;
>
> #ifndef CONFIG_CPU_V7M
> @@ -470,7 +471,10 @@ void notrace cpu_init(void)
> "msr cpsr_c, %5\n\t"
> "add r14, %0, %6\n\t"
> "mov sp, r14\n\t"
> - "msr cpsr_c, %7"
> + "msr cpsr_c, %7\n\t"
> + "add r14, %0, %8\n\t"
> + "mov sp, r14\n\t"
> + "msr cpsr_c, %9"
> :
> : "r" (stk),
> PLC (PSR_F_BIT | PSR_I_BIT | IRQ_MODE),
> @@ -479,6 +483,8 @@ void notrace cpu_init(void)
> "I" (offsetof(struct stack, abt[0])),
> PLC (PSR_F_BIT | PSR_I_BIT | UND_MODE),
> "I" (offsetof(struct stack, und[0])),
> + PLC (PSR_F_BIT | PSR_I_BIT | FIQ_MODE),
> + "I" (offsetof(struct stack, fiq[0])),
> PLC (PSR_F_BIT | PSR_I_BIT | SVC_MODE)
> : "r14");
> #endif
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index c8e4bb7..7912a9e 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -25,6 +25,7 @@
> #include <linux/delay.h>
> #include <linux/init.h>
> #include <linux/sched.h>
> +#include <linux/irq.h>
>
> #include <linux/atomic.h>
> #include <asm/cacheflush.h>
> @@ -460,10 +461,29 @@ die_sig:
> arm_notify_die("Oops - undefined instruction", regs, &info, 0, 6);
> }
>
> -asmlinkage void do_unexp_fiq (struct pt_regs *regs)
> +/*
> + * Handle FIQ similarly to NMI on x86 systems.
> + *
> + * The runtime environment for NMIs is extremely restrictive
> + * (NMIs can pre-empt critical sections meaning almost all locking is
> + * forbidden) meaning this default FIQ handling must only be used in
> + * circumstances where non-maskability improves robustness, such as
> + * watchdog or debug logic.
> + *
> + * This handler is not appropriate for general purpose use in drivers
> + * platform code and can be overrideen using set_fiq_handler.
> + */
> +asmlinkage void __exception_irq_entry handle_fiq_as_nmi(struct pt_regs *regs)
> {
> - printk("Hmm. Unexpected FIQ received, but trying to continue\n");
> - printk("You may have a hardware problem...\n");
> +#ifdef CONFIG_FIQ
> + struct pt_regs *old_regs = set_irq_regs(regs);
> +
> + nmi_enter();
> + /* nop for now */
> + nmi_exit();
> +
> + set_irq_regs(old_regs);
> +#endif
> }
>
> /*
> --
> 1.9.3
>
>
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