Re: [PATCH v2] powerpc/32: Add support for out-of-line static calls
Hi Peter, Le 31/08/2021 à 16:00, Peter Zijlstra a écrit : On Tue, Aug 31, 2021 at 01:12:26PM +, Christophe Leroy wrote: Yes, this should work nicely! Since you have the two nop's at the end, you could frob in an optimization for __static_call_return0 without too much issue. Replace the two nops with (excuse my ppc asm): li r3, 0 blr and augment arch_static_call_transform() with something like: if (func == &__static_call_return0) err = patch_branch(tramp, tramp+24, 0); I just discovered that we likely have an issue with the implementation of that RET0 static call. Looking at System.map I have: c0004fc0 t __static_call_return0 c0011518 t __static_call_return0 c00d8160 t __static_call_return0 So when we do: if (func == &__static_call_return0) It is unlikely that we'll get the expected one. I see __static_call_return0 is defined as 'static inline' in include/linux/static_call.h Any reason for not having it as a single global symbol instead ? Thanks Christophe
Re: [PATCH v2] powerpc/32: Add support for out-of-line static calls
On Tue, Aug 31, 2021 at 01:12:26PM +, Christophe Leroy wrote:
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 36b72d972568..a0fe69d8ec83 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -247,6 +247,7 @@ config PPC
> select HAVE_SOFTIRQ_ON_OWN_STACK
> select HAVE_STACKPROTECTOR if PPC32 &&
> $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2)
> select HAVE_STACKPROTECTOR if PPC64 &&
> $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r13)
> + select HAVE_STATIC_CALL if PPC32
> select HAVE_SYSCALL_TRACEPOINTS
> select HAVE_VIRT_CPU_ACCOUNTING
> select HUGETLB_PAGE_SIZE_VARIABLE if PPC_BOOK3S_64 && HUGETLB_PAGE
> diff --git a/arch/powerpc/include/asm/static_call.h
> b/arch/powerpc/include/asm/static_call.h
> new file mode 100644
> index ..2402c6d32439
> --- /dev/null
> +++ b/arch/powerpc/include/asm/static_call.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_POWERPC_STATIC_CALL_H
> +#define _ASM_POWERPC_STATIC_CALL_H
> +
> +#define __POWERPC_SCT(name, inst)\
> + asm(".pushsection .text, \"ax\" \n" \
> + ".align 5 \n" \
> + ".globl " STATIC_CALL_TRAMP_STR(name) " \n" \
> + STATIC_CALL_TRAMP_STR(name) ": \n" \
> + inst " \n" \
> + " lis 12,1f@ha\n" \
> + " lwz 12,1f@l(12) \n" \
> + " mtctr 12 \n" \
> + " bctr\n" \
> + "1: .long 0 \n" \
> + " nop \n" \
> + " nop \n" \
> + ".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
> + ".size " STATIC_CALL_TRAMP_STR(name) ", . - "
> STATIC_CALL_TRAMP_STR(name) " \n" \
> + ".popsection\n")
> +
> +#define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)__POWERPC_SCT(name, "b
> " #func)
> +#define ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name) __POWERPC_SCT(name,
> "blr")
> +
> +#endif /* _ASM_POWERPC_STATIC_CALL_H */
> diff --git a/arch/powerpc/kernel/static_call.c
> b/arch/powerpc/kernel/static_call.c
> new file mode 100644
> index ..e5e78205ccb4
> --- /dev/null
> +++ b/arch/powerpc/kernel/static_call.c
> @@ -0,0 +1,36 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include
> +#include
> +
> +#include
> +
> +void arch_static_call_transform(void *site, void *tramp, void *func, bool
> tail)
> +{
> + int err;
> + unsigned long target = (long)func;
> + bool is_short = is_offset_in_branch_range((long)target - (long)tramp);
> +
> + if (!tramp)
> + return;
> +
> + mutex_lock(&text_mutex);
> +
> + if (func && !is_short) {
> + err = patch_instruction(tramp + 20, ppc_inst(target));
> + if (err)
> + goto out;
> + }
> +
> + if (!func)
> + err = patch_instruction(tramp, ppc_inst(PPC_RAW_BLR()));
> + else if (is_short)
> + err = patch_branch(tramp, target, 0);
> + else
> + err = patch_instruction(tramp, ppc_inst(PPC_RAW_NOP()));
> +out:
> + mutex_unlock(&text_mutex);
> +
> + if (err)
> + panic("%s: patching failed %pS at %pS\n", __func__, func,
> tramp);
> +}
> +EXPORT_SYMBOL_GPL(arch_static_call_transform);
Yes, this should work nicely!
Since you have the two nop's at the end, you could frob in an
optimization for __static_call_return0 without too much issue.
Replace the two nops with (excuse my ppc asm):
li r3, 0
blr
and augment arch_static_call_transform() with something like:
if (func == &__static_call_return0)
err = patch_branch(tramp, tramp+24, 0);
[PATCH v2] powerpc/32: Add support for out-of-line static calls
Add support for out-of-line static calls on PPC32. This change improve performance of calls to global function pointers by using direct calls instead of indirect calls. The trampoline is initialy populated with a 'blr' or branch to target, followed by an unreachable long jump sequence. In order to cater with parallele execution, the trampoline needs to be updated in a way that ensures it remains consistent at all time. This means we can't use the traditional lis/addi to load r12 with the target address, otherwise there would be a window during which the first instruction contains the upper part of the new target address while the second instruction still contains the lower part of the old target address. To avoid that the target address is stored just after the 'bctr' and loaded from there with a single instruction. Then, depending on the target distance, arch_static_call_transform() will either replace the first instruction by a direct 'bl ' or 'nop' in order to have the trampoline fall through the long jump sequence. Performancewise the long jump sequence is probably not better than the indirect calls set by GCC when we don't use static calls, but such calls are unlikely to be required on powerpc32: With most configurations the kernel size is far below 32 Mbytes so only modules may happen to be too far. And even modules are likely to be close enough as they are allocated below the kernel core and as close as possible of the kernel text. static_call selftest is running successfully with this change. With this patch, __do_irq() has the following sequence to trace irq entries: c0004a00 <__SCT__tp_func_irq_entry>: c0004a00: 48 00 00 e0 b c0004ae0 <__traceiter_irq_entry> c0004a04: 3d 80 c0 00 lis r12,-16384 c0004a08: 81 8c 4a 14 lwz r12,18964(r12) c0004a0c: 7d 89 03 a6 mtctr r12 c0004a10: 4e 80 04 20 bctr c0004a14: 00 00 00 00 .long 0x0 c0004a18: 60 00 00 00 nop c0004a1c: 60 00 00 00 nop ... c0005654 <__do_irq>: ... c0005664: 7c 7f 1b 78 mr r31,r3 ... c00056a0: 81 22 00 00 lwz r9,0(r2) c00056a4: 39 29 00 01 addir9,r9,1 c00056a8: 91 22 00 00 stw r9,0(r2) c00056ac: 3d 20 c0 af lis r9,-16209 c00056b0: 81 29 74 cc lwz r9,29900(r9) c00056b4: 2c 09 00 00 cmpwi r9,0 c00056b8: 41 82 00 10 beq c00056c8 <__do_irq+0x74> c00056bc: 80 69 00 04 lwz r3,4(r9) c00056c0: 7f e4 fb 78 mr r4,r31 c00056c4: 4b ff f3 3d bl c0004a00 <__SCT__tp_func_irq_entry> Before this patch, __do_irq() was doing the following to trace irq entries: c0005700 <__do_irq>: ... c0005710: 7c 7e 1b 78 mr r30,r3 ... c000574c: 93 e1 00 0c stw r31,12(r1) c0005750: 81 22 00 00 lwz r9,0(r2) c0005754: 39 29 00 01 addir9,r9,1 c0005758: 91 22 00 00 stw r9,0(r2) c000575c: 3d 20 c0 af lis r9,-16209 c0005760: 83 e9 f4 cc lwz r31,-2868(r9) c0005764: 2c 1f 00 00 cmpwi r31,0 c0005768: 41 82 00 24 beq c000578c <__do_irq+0x8c> c000576c: 81 3f 00 00 lwz r9,0(r31) c0005770: 80 7f 00 04 lwz r3,4(r31) c0005774: 7d 29 03 a6 mtctr r9 c0005778: 7f c4 f3 78 mr r4,r30 c000577c: 4e 80 04 21 bctrl c0005780: 85 3f 00 0c lwzur9,12(r31) c0005784: 2c 09 00 00 cmpwi r9,0 c0005788: 40 82 ff e4 bne c000576c <__do_irq+0x6c> Behind the fact of now using a direct 'bl' instead of a 'load/mtctr/bctr' sequence, we can also see that we get one less register on the stack. Signed-off-by: Christophe Leroy --- v2: Use indirect load in long jump sequence to cater with parallele execution and preemption. --- arch/powerpc/Kconfig | 1 + arch/powerpc/include/asm/static_call.h | 25 ++ arch/powerpc/kernel/Makefile | 2 +- arch/powerpc/kernel/static_call.c | 36 ++ 4 files changed, 63 insertions(+), 1 deletion(-) create mode 100644 arch/powerpc/include/asm/static_call.h create mode 100644 arch/powerpc/kernel/static_call.c diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig index 36b72d972568..a0fe69d8ec83 100644 --- a/arch/powerpc/Kconfig +++ b/arch/powerpc/Kconfig @@ -247,6 +247,7 @@ config PPC select HAVE_SOFTIRQ_ON_OWN_STACK select HAVE_STACKPROTECTOR if PPC32 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2) select HAVE_STACKPROTECTOR if PPC64 && $(cc-
