On Mon, 11 Nov 2013, Stephen Boyd wrote:
> On 11/09/13 21:03, Nicolas Pitre wrote:
> > Bah..... NAK. We are doing runtime patching of the kernel for many
> > many things already. So why not do the same here?
>
> static keys are a form of runtime patching, albeit not as extreme as
> you're suggesting.
>
> >
> > The obvious strategy is to simply overwrite the start of the existing
> > __aeabi_idiv code with the "sdiv r0, r0, r1" and "bx lr" opcodes.
> >
> > Similarly for the unsigned case.
>
> I was thinking the same thing when I wrote this, but I didn't know how
> to tell the compiler to either inline this function or to let me inilne
> an assembly stub with some section magic.
>
> >
> > That let you test the hardware capability only once during boot instead
> > of everytime a divide operation is performed.
>
> The test for hardware capability really isn't done more than once during
> boot. The assembly is like so at compile time
>
> 00000000 <__aeabi_idiv>:
> 0: nop {0}
> 4: b 0 <___aeabi_idiv>
> 8: sdiv r0, r0, r1
> c: bx lr
>
> and after we test and find support for the instruction it will be
> replaced with
>
> 00000000 <__aeabi_idiv>:
> 0: b 8
> 4: b 0 <___aeabi_idiv>
> 8: sdiv r0, r0, r1
> c: bx lr
>
> Unfortunately we still have to jump to this function. It would be great
> if we could inline this function at the call site but as I already said
> I don't know how to do that.
What about this patch which I think is currently your best option. Note
it would need to use the facilities from asm/opcodes.h to make it endian
agnostic.
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index 6a1b8a81b1..379cffe4ab 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -383,6 +383,34 @@ static void __init cpuid_init_hwcaps(void)
elf_hwcap |= HWCAP_IDIVT;
}
+ /*
+ * Patch our division routines with the corresponding opcode
+ * if the hardware supports it.
+ */
+ if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVT)) {
+ extern char __aeabi_uidiv, __aeabi_idiv;
+ u16 *uidiv = (u16 *)&__aeabi_uidiv;
+ u16 *idiv = (u16 *)&__aeabi_idiv;
+
+ uidiv[0] = 0xfbb0; /* udiv r0, r0, r1 */
+ uidiv[1] = 0xf0f1;
+ uidiv[2] = 0x4770; /* bx lr */
+
+ idiv[0] = 0xfb90; /* sdiv r0, r0, r1 */
+ idiv[1] = 0xf0f1;
+ idiv[2] = 0x4770; /* bx lr */
+ } else if (!IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap &
HWCAP_IDIVA)) {
+ extern char __aeabi_uidiv, __aeabi_idiv;
+ u32 *uidiv = (u32 *)&__aeabi_uidiv;
+ u32 *idiv = (u32 *)&__aeabi_idiv;
+
+ uidiv[0] = 0xe730f110; /* udiv r0, r0, r1 */
+ uidiv[1] = 0xe12fff1e; /* bx lr */
+
+ idiv[0] = 0xe710f110; /* sdiv r0, r0, r1 */
+ idiv[1] = 0xe12fff1e; /* bx lr */
+ }
+
/* LPAE implies atomic ldrd/strd instructions */
vmsa = (read_cpuid_ext(CPUID_EXT_MMFR0) & 0xf) >> 0;
if (vmsa >= 5)
Nicolas
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