Subject: Re: [PATCH] powerpc: Fix "attempt to move .org backwards" error

From: Mike Qiu <[email-masked]>
Date: Sat, 27 Apr 2013 17:34:25 +0800

于 2013/4/26 11:51, Paul Mackerras 写道:
> Building a 64-bit powerpc kernel with PR KVM enabled currently gives
> this error:
>
>    AS      arch/powerpc/kernel/head_64.o
> arch/powerpc/kernel/exceptions-64s.S: Assembler messages:
> arch/powerpc/kernel/exceptions-64s.S:258: Error: attempt to move .org backwards
> make[2]: *** [arch/powerpc/kernel/head_64.o] Error 1
>
> This happens because the MASKABLE_EXCEPTION_PSERIES macro turns into
> 33 instructions, but we only have space for 32 at the decrementer
> interrupt vector (from 0x900 to 0x980).
>
> In the code generated by the MASKABLE_EXCEPTION_PSERIES macro, we
> currently have two instances of the HMT_MEDIUM macro, which has the
> effect of setting the SMT thread priority to medium.  One is the
> first instruction, and is overwritten by a no-op on processors where
> we save the PPR (processor priority register), that is, POWER7 or
> later.  The other is after we have saved the PPR.
>
> In order to reduce the code at 0x900 by one instruction, we omit the
> first HMT_MEDIUM.  On processors without SMT this will have no effect
> since HMT_MEDIUM is a no-op there.  On POWER5 and RS64 machines this
> will mean that the first few instructions take a little longer in the
> case where a decrementer interrupt occurs when the hardware thread is
> running at low SMT priority.  On POWER6 and later machines, the
> hardware automatically boosts the thread priority when a decrementer
> interrupt is taken if the thread priority was below medium, so this
> change won't make any difference.
>
> The alternative would be to branch out of line after saving the CFAR.
> However, that would incur an extra overhead on all processors, whereas
> the approach adopted here only adds overhead on older threaded processors.
>
> Signed-off-by: Paul Mackerras <[email-masked]>
> ---
>   arch/powerpc/include/asm/exception-64s.h |    2 +-
>   arch/powerpc/kernel/exceptions-64s.S     |    7 ++++++-
>   2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
> index 05e6d2e..8e5fae8 100644
> --- a/arch/powerpc/include/asm/exception-64s.h
> +++ b/arch/powerpc/include/asm/exception-64s.h
> @@ -414,7 +414,6 @@ label##_relon_hv:						\
>   #define SOFTEN_NOTEST_HV(vec)		_SOFTEN_TEST(EXC_HV, vec)
>
>   #define __MASKABLE_EXCEPTION_PSERIES(vec, label, h, extra)		\
> -	HMT_MEDIUM_PPR_DISCARD;						\
>   	SET_SCRATCH0(r13);    /* save r13 */				\
>   	EXCEPTION_PROLOG_0(PACA_EXGEN);					\
>   	__EXCEPTION_PROLOG_1(PACA_EXGEN, extra, vec);			\
> @@ -427,6 +426,7 @@ label##_relon_hv:						\
>   	. = loc;							\
>   	.globl label##_pSeries;						\
>   label##_pSeries:							\
> +	HMT_MEDIUM_PPR_DISCARD;						\
>   	_MASKABLE_EXCEPTION_PSERIES(vec, label,				\
>   				    EXC_STD, SOFTEN_TEST_PR)
>
> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index 56bd923..574db3f 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -235,6 +235,7 @@ instruction_access_slb_pSeries:
>   	.globl hardware_interrupt_hv;
>   hardware_interrupt_pSeries:
>   hardware_interrupt_hv:
> +	HMT_MEDIUM_PPR_DISCARD
>   	BEGIN_FTR_SECTION
>   		_MASKABLE_EXCEPTION_PSERIES(0x502, hardware_interrupt,
>   					    EXC_HV, SOFTEN_TEST_HV)
> @@ -254,7 +255,11 @@ hardware_interrupt_hv:
>   	STD_EXCEPTION_PSERIES(0x800, 0x800, fp_unavailable)
>   	KVM_HANDLER_PR(PACA_EXGEN, EXC_STD, 0x800)
>
> -	MASKABLE_EXCEPTION_PSERIES(0x900, 0x900, decrementer)
> +	. = 0x900
> +	.globl decrementer_pSeries
> +decrementer_pSeries:
> +	_MASKABLE_EXCEPTION_PSERIES(0x900, decrementer, EXC_STD, SOFTEN_TEST_PR)
> +
>   	STD_EXCEPTION_HV(0x980, 0x982, hdecrementer)
>
>   	MASKABLE_EXCEPTION_PSERIES(0xa00, 0xa00, doorbell_super)
test-by: Mike Qiu <[email-masked]>
It's workable for me. but I just use this patch to compile and boot up 
the machine. not do any performance test:)