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Message-ID: <4A9428AE.8020209@goop.org>
Date: Tue, 25 Aug 2009 11:08:46 -0700
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Pekka Enberg <penberg@...helsinki.fi>
CC: Arnd Hannemann <hannemann@...s.rwth-aachen.de>,
Arnd Hannemann <Arnd.Hannemann@...s.rwth-aachen.de>,
LKML <linux-kernel@...r.kernel.org>,
"hannes@...xchg.org" <hannes@...xchg.org>,
"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
Ingo Molnar <mingo@...e.hu>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [bisected] 2.6.31 regression: fails to boot as xen guest
On 08/25/09 11:03, Pekka Enberg wrote:
> On Tue, 2009-08-25 at 19:49 +0200, Arnd Hannemann wrote:
>
>> Hi Pekka,
>>
>> Pekka Enberg wrote:
>>
>>> On Tue, 2009-08-25 at 18:49 +0200, Arnd Hannemann wrote:
>>>
>>>>> Thanks for doing the bisect! Can we also see your .config also?
>>>>>
>>>> Config for -rc7 is attached. My bisect configs were based on that
>>>>
>>> Thanks! While we wait for the Xen people, you can try the following
>>> patch to see if we can narrow the bug down to trap_init().
>>>
>> Yes seems to be trap_init().
>> -rc7 with this patch applied boots up to the prompt.
>>
> Thanks for testing! Ingo, what do you think of the following patch?
> AFAICT, x86-32 is the only architecture playing with traps in mem_init()
> so this should be the safest fix for 2.6.31.
>
Huh, interesting. I wonder if this is the same as the problem I'd been
chasing or separate...
J
> Pekka
>
> >From b739e3c3baa6312664b4ea676bdf73df27fcecbc Mon Sep 17 00:00:00 2001
> From: Pekka Enberg <penberg@...helsinki.fi>
> Date: Tue, 25 Aug 2009 20:55:25 +0300
> Subject: [PATCH] x86: Move WP bit testing to trap_init()
>
> Commit 83b519e8b9572c319c8e0c615ee5dd7272856090 ("slab: setup allocators
> earlier in the boot sequence") moved trap_init() earlier in the boot
> sequence to avoid a hard crash with 32-bit x86 in mem_init().
>
> Unfortunately the change broke Xen so make trap_init() later and move
> the WP bit test from mem_init() to trap_init().
>
> Signed-off-by: Pekka Enberg <penberg@...helsinki.fi>
> ---
> arch/x86/kernel/traps.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++
> arch/x86/mm/init_32.c | 56 ----------------------------------------------
> init/main.c | 2 +-
> 3 files changed, 58 insertions(+), 57 deletions(-)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 5204332..2084408 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -906,6 +906,60 @@ dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code)
> return;
> do_trap(32, SIGILL, "iret exception", regs, error_code, &info);
> }
> +
> +static noinline int do_test_wp_bit(void);
> +
> +/*
> + * Test if the WP bit works in supervisor mode. It isn't supported on 386's
> + * and also on some strange 486's. All 586+'s are OK. This used to involve
> + * black magic jumps to work around some nasty CPU bugs, but fortunately the
> + * switch to using exceptions got rid of all that.
> + */
> +static void __init test_wp_bit(void)
> +{
> + printk(KERN_INFO
> + "Checking if this processor honours the WP bit even in supervisor mode...");
> +
> + /* Any page-aligned address will do, the test is non-destructive */
> + __set_fixmap(FIX_WP_TEST, __pa(&swapper_pg_dir), PAGE_READONLY);
> + boot_cpu_data.wp_works_ok = do_test_wp_bit();
> + clear_fixmap(FIX_WP_TEST);
> +
> + if (!boot_cpu_data.wp_works_ok) {
> + printk(KERN_CONT "No.\n");
> +#ifdef CONFIG_X86_WP_WORKS_OK
> + panic(
> + "This kernel doesn't support CPU's with broken WP. Recompile it for a 386!");
> +#endif
> + } else {
> + printk(KERN_CONT "Ok.\n");
> + }
> +}
> +
> +/*
> + * This function cannot be __init, since exceptions don't work in that
> + * section. Put this after the callers, so that it cannot be inlined.
> + */
> +static noinline int do_test_wp_bit(void)
> +{
> + char tmp_reg;
> + int flag;
> +
> + __asm__ __volatile__(
> + " movb %0, %1 \n"
> + "1: movb %1, %0 \n"
> + " xorl %2, %2 \n"
> + "2: \n"
> + _ASM_EXTABLE(1b,2b)
> + :"=m" (*(char *)fix_to_virt(FIX_WP_TEST)),
> + "=q" (tmp_reg),
> + "=r" (flag)
> + :"2" (1)
> + :"memory");
> +
> + return flag;
> +}
> +
> #endif
>
> void __init trap_init(void)
> @@ -982,5 +1036,8 @@ void __init trap_init(void)
>
> #ifdef CONFIG_X86_32
> x86_quirk_trap_init();
> +
> + if (boot_cpu_data.wp_works_ok < 0)
> + test_wp_bit();
> #endif
> }
> diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> index 3cd7711..e22bb8a 100644
> --- a/arch/x86/mm/init_32.c
> +++ b/arch/x86/mm/init_32.c
> @@ -54,8 +54,6 @@
>
> unsigned long highstart_pfn, highend_pfn;
>
> -static noinline int do_test_wp_bit(void);
> -
> bool __read_mostly __vmalloc_start_set = false;
>
> static __init void *alloc_low_page(void)
> @@ -830,33 +828,6 @@ void __init paging_init(void)
> zone_sizes_init();
> }
>
> -/*
> - * Test if the WP bit works in supervisor mode. It isn't supported on 386's
> - * and also on some strange 486's. All 586+'s are OK. This used to involve
> - * black magic jumps to work around some nasty CPU bugs, but fortunately the
> - * switch to using exceptions got rid of all that.
> - */
> -static void __init test_wp_bit(void)
> -{
> - printk(KERN_INFO
> - "Checking if this processor honours the WP bit even in supervisor mode...");
> -
> - /* Any page-aligned address will do, the test is non-destructive */
> - __set_fixmap(FIX_WP_TEST, __pa(&swapper_pg_dir), PAGE_READONLY);
> - boot_cpu_data.wp_works_ok = do_test_wp_bit();
> - clear_fixmap(FIX_WP_TEST);
> -
> - if (!boot_cpu_data.wp_works_ok) {
> - printk(KERN_CONT "No.\n");
> -#ifdef CONFIG_X86_WP_WORKS_OK
> - panic(
> - "This kernel doesn't support CPU's with broken WP. Recompile it for a 386!");
> -#endif
> - } else {
> - printk(KERN_CONT "Ok.\n");
> - }
> -}
> -
> static struct kcore_list kcore_mem, kcore_vmalloc;
>
> void __init mem_init(void)
> @@ -956,9 +927,6 @@ void __init mem_init(void)
> BUG_ON(VMALLOC_START >= VMALLOC_END);
> BUG_ON((unsigned long)high_memory > VMALLOC_START);
>
> - if (boot_cpu_data.wp_works_ok < 0)
> - test_wp_bit();
> -
> save_pg_dir();
> zap_low_mappings(true);
> }
> @@ -975,30 +943,6 @@ int arch_add_memory(int nid, u64 start, u64 size)
> }
> #endif
>
> -/*
> - * This function cannot be __init, since exceptions don't work in that
> - * section. Put this after the callers, so that it cannot be inlined.
> - */
> -static noinline int do_test_wp_bit(void)
> -{
> - char tmp_reg;
> - int flag;
> -
> - __asm__ __volatile__(
> - " movb %0, %1 \n"
> - "1: movb %1, %0 \n"
> - " xorl %2, %2 \n"
> - "2: \n"
> - _ASM_EXTABLE(1b,2b)
> - :"=m" (*(char *)fix_to_virt(FIX_WP_TEST)),
> - "=q" (tmp_reg),
> - "=r" (flag)
> - :"2" (1)
> - :"memory");
> -
> - return flag;
> -}
> -
> #ifdef CONFIG_DEBUG_RODATA
> const int rodata_test_data = 0xC3;
> EXPORT_SYMBOL_GPL(rodata_test_data);
> diff --git a/init/main.c b/init/main.c
> index 2d9d6bd..5c4dacb 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -603,7 +603,6 @@ asmlinkage void __init start_kernel(void)
> pidhash_init();
> vfs_caches_init_early();
> sort_main_extable();
> - trap_init();
> mm_init();
> /*
> * Set up the scheduler prior starting any interrupts (such as the
> @@ -621,6 +620,7 @@ asmlinkage void __init start_kernel(void)
> "enabled *very* early, fixing it\n");
> local_irq_disable();
> }
> + trap_init();
> rcu_init();
> /* init some links before init_ISA_irqs() */
> early_irq_init();
>
--
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