[PATCH v3 1/5] ARM: protect usage of cr_alignment by #ifdef CONFIG_CPU_CP15
Uwe Kleine-König
u.kleine-koenig at pengutronix.de
Wed Mar 14 15:51:20 EDT 2012
On Wed, Mar 14, 2012 at 11:24:36AM -0400, Nicolas Pitre wrote:
> On Wed, 14 Mar 2012, Uwe Kleine-König wrote:
>
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
>
> I think you could reduce the amount of #ifdef's significantly here.
> While the cr_alignment variable and its sister are of no use in a
> no-CP15 system, you could still leave them there initialized to zero and
> only conditionally compile out the CP15 modifyers with a dummy version.
> This is only 2 words wasted in your kernel image to keep the code
> cleaner.
The main purpose to create this patch is that for v7m I don't compile
entry-armv.S. As cr_alignment is defined there I don't "have" this
variable. So I have to move the definition to a different file to waste
it :-)
> More comments below.
>
> > index 854bd22..efeb2d0 100644
> > --- a/arch/arm/kernel/head-common.S
> > +++ b/arch/arm/kernel/head-common.S
> > @@ -98,8 +98,10 @@ __mmap_switched:
> > str r9, [r4] @ Save processor ID
> > str r1, [r5] @ Save machine type
> > str r2, [r6] @ Save atags pointer
> > - bic r4, r0, #CR_A @ Clear 'A' bit
> > - stmia r7, {r0, r4} @ Save control register values
> > + cmp r7, #0
> > + itt ne
> > + bicne r4, r0, #CR_A @ Clear 'A' bit
> > + stmneia r7, {r0, r4} @ Save control register values
>
> The kernel is compiled with -mimplicit-it so you do not need to specify
> any it here.
ok
> > index a255c39..50d3df8 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -472,9 +472,13 @@ static void __init setup_processor(void)
> > cpu_cache = *list->cache;
> > #endif
> >
> > - printk("CPU: %s [%08x] revision %d (ARMv%s), cr=%08lx\n",
> > + printk("CPU: %s [%08x] revision %d (ARMv%s)",
> > cpu_name, read_cpuid_id(), read_cpuid_id() & 15,
> > - proc_arch[cpu_architecture()], cr_alignment);
> > + proc_arch[cpu_architecture()]);
> > +
> > +#ifdef CONFIG_CPU_CP15
> > + printk(KERN_CONT ", cr=%08lx\n", cr_alignment);
> > +#endif
> >
>
> Again, you could just leave the original display, with cr=00000000 which
> is a fairly good representation of reality.
Maybe having
#define cr_alignment 0
for the !CONFIG_CPU_CP15 case would be a nice alternative. This way all
places that want to modify cr_alignment fail to compile, but reading
gives a "fairly good representation of reality".
Having said that I'm not sure about "fairly good". v7m also supports
unaligned accesses. But it's not configured in a cp15 register
(obviously) but in a system register. Quoting ARMARM-v7m:
Configuration and Control Register, CCR
[...]
Bit [3] UNALIGN_TRP Controls the trapping of unaligned word or
halfword accesses:
0 = Trapping disabled.
1 = Trapping enabled.
Unaligned load-store multiples and word
or halfword exclusive accesses always
fault.
So we need a more general abstraction to have correct and clean code?
But note I didn't check the two different implementations deeply to be
sure not to compare two different things.
> > diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
> > index caf14dc..119d178 100644
> > --- a/arch/arm/mm/alignment.c
> > +++ b/arch/arm/mm/alignment.c
> > @@ -89,7 +89,11 @@ core_param(alignment, ai_usermode, int, 0600);
> > /* Return true if and only if the ARMv6 unaligned access model is in use. */
> > static bool cpu_is_v6_unaligned(void)
> > {
> > +#ifdef CONFIG_CPU_CP15
> > return cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U);
> > +#else
> > + return 0;
> > +#endif
> > }
>
> Same here. With cr_alignment set to zero, you don't need the above
> #ifdef.
>
> > static int safe_usermode(int new_usermode, bool warn)
> > @@ -961,12 +965,14 @@ static int __init alignment_init(void)
> > return -ENOMEM;
> > #endif
> >
> > +#ifdef CONFIG_CPU_CP15
> > if (cpu_is_v6_unaligned()) {
> > cr_alignment &= ~CR_A;
> > cr_no_alignment &= ~CR_A;
> > set_cr(cr_alignment);
> > ai_usermode = safe_usermode(ai_usermode, false);
> > }
> > +#endif
>
> Here the #ifdef is probably legitimate.
>
> > hook_fault_code(FAULT_CODE_ALIGNMENT, do_alignment, SIGBUS, BUS_ADRALN,
> > "alignment exception");
> > diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> > index 94c5a0c..f6dbe1a 100644
> > --- a/arch/arm/mm/mmu.c
> > +++ b/arch/arm/mm/mmu.c
> > @@ -109,8 +109,10 @@ static int __init early_cachepolicy(char *p)
> >
> > if (memcmp(p, cache_policies[i].policy, len) == 0) {
> > cachepolicy = i;
> > +#ifdef CONFIG_CPU_CP15
> > cr_alignment &= ~cache_policies[i].cr_mask;
> > cr_no_alignment &= ~cache_policies[i].cr_mask;
> > +#endif
>
> Probably here as well.
>
> > break;
> > }
> > }
> > @@ -128,7 +130,9 @@ static int __init early_cachepolicy(char *p)
> > cachepolicy = CPOLICY_WRITEBACK;
> > }
> > flush_cache_all();
> > +#ifdef CONFIG_CPU_CP15
> > set_cr(cr_alignment);
> > +#endif
>
> However it might be best to provide a dummy set_cr() instead of
> #ifdef'ing it out everywhere.
What should the dummy set_cr do? Print a runtime warning if called with
!= 0?
Best regards
Uwe
--
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