Messages in this thread | | | From | Michael Ellerman <> | Subject | Re: [PATCH v2 1/8] powerpc/32: Add VDSO version of getcpu | Date | Wed, 18 Sep 2019 15:51:54 +1000 |
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Hi Christophe,
Sorry I'm late replying to this.
Christophe Leroy <christophe.leroy@c-s.fr> writes: > Commit 18ad51dd342a ("powerpc: Add VDSO version of getcpu") added > getcpu() for PPC64 only, by making use of a user readable general > purpose SPR. > > PPC32 doesn't have any such SPR, a full system call can still be > avoided by implementing a fast system call which reads the CPU id > in the task struct and returns immediately without going back in > virtual mode. > > Before the patch, vdsotest reported: > getcpu: syscall: 1572 nsec/call > getcpu: libc: 1787 nsec/call > getcpu: vdso: not tested > > Now, vdsotest reports: > getcpu: syscall: 1582 nsec/call > getcpu: libc: 667 nsec/call > getcpu: vdso: 368 nsec/call > > For non SMP, just return CPU id 0 from the VDSO directly. > > PPC32 doesn't support CONFIG_NUMA so NUMA node is always 0. > > Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr> > > --- > v2: fixed build error in getcpu.S > --- > arch/powerpc/include/asm/vdso.h | 2 ++ > arch/powerpc/kernel/head_32.h | 13 +++++++++++++ > arch/powerpc/kernel/head_booke.h | 11 +++++++++++ > arch/powerpc/kernel/vdso32/Makefile | 4 +--- > arch/powerpc/kernel/vdso32/getcpu.S | 7 +++++++ > arch/powerpc/kernel/vdso32/vdso32.lds.S | 2 -- > 6 files changed, 34 insertions(+), 5 deletions(-) > > diff --git a/arch/powerpc/include/asm/vdso.h b/arch/powerpc/include/asm/vdso.h > index b5e1f8f8a05c..adb54782df5f 100644 > --- a/arch/powerpc/include/asm/vdso.h > +++ b/arch/powerpc/include/asm/vdso.h > @@ -16,6 +16,8 @@ > /* Define if 64 bits VDSO has procedure descriptors */ > #undef VDS64_HAS_DESCRIPTORS > > +#define NR_MAGIC_FAST_VDSO_SYSCALL 0x789a
We are still in the middle of the years long process of removing the "magic" syscall on 64-bit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/exceptions-64s.S?commit=4d856f72c10ecb060868ed10ff1b1453943fc6c8#n1578
Can we not add another one on 32-bit?
Is it really such a fast path that it's worth putting a wart in the syscall entry like that?
Is there some other method? On s390 they have a per-cpu VDSO page, that would be a nice option. How we do that would be specific to a particular MMU, and maybe not even possible with some MMUs. So maybe that's not feasible.
If you do want to add a fastpath syscall then please just add it as a regular syscall number, that way it's at least a bit less of a wart. It's still not visible via tracing/ptrace etc. which is a pain but at least the number is not "magical" too.
cheers
> /* Offsets relative to thread->vdso_base */ > diff --git a/arch/powerpc/kernel/head_32.h b/arch/powerpc/kernel/head_32.h > index 4a692553651f..a2e38b59785a 100644 > --- a/arch/powerpc/kernel/head_32.h > +++ b/arch/powerpc/kernel/head_32.h > @@ -3,6 +3,8 @@ > #define __HEAD_32_H__ > > #include <asm/ptrace.h> /* for STACK_FRAME_REGS_MARKER */ > +#include <asm/vdso.h> > +#include <asm/asm-offsets.h> > > /* > * MSR_KERNEL is > 0x8000 on 4xx/Book-E since it include MSR_CE. > @@ -74,7 +76,13 @@ > .endm > > .macro SYSCALL_ENTRY trapno > +#ifdef CONFIG_SMP > + cmplwi cr0, r0, NR_MAGIC_FAST_VDSO_SYSCALL > +#endif > mfspr r12,SPRN_SPRG_THREAD > +#ifdef CONFIG_SMP > + beq- 1f > +#endif > mfcr r10 > lwz r11,TASK_STACK-THREAD(r12) > mflr r9 > @@ -152,6 +160,11 @@ > mtspr SPRN_SRR0,r11 > SYNC > RFI /* jump to handler, enable MMU */ > +#ifdef CONFIG_SMP > +1: > + lwz r5, TASK_CPU - THREAD(r12) > + RFI > +#endif > .endm > > /* > diff --git a/arch/powerpc/kernel/head_booke.h b/arch/powerpc/kernel/head_booke.h > index 2ae635df9026..c534e87cac84 100644 > --- a/arch/powerpc/kernel/head_booke.h > +++ b/arch/powerpc/kernel/head_booke.h > @@ -3,6 +3,8 @@ > #define __HEAD_BOOKE_H__ > > #include <asm/ptrace.h> /* for STACK_FRAME_REGS_MARKER */ > +#include <asm/vdso.h> > +#include <asm/asm-offsets.h> > #include <asm/kvm_asm.h> > #include <asm/kvm_booke_hv_asm.h> > > @@ -104,6 +106,10 @@ FTR_SECTION_ELSE > #ifdef CONFIG_KVM_BOOKE_HV > ALT_FTR_SECTION_END_IFSET(CPU_FTR_EMB_HV) > #endif > +#ifdef CONFIG_SMP > + cmplwi cr0, r0, NR_MAGIC_FAST_VDSO_SYSCALL > + beq- 1f > +#endif > BOOKE_CLEAR_BTB(r11) > lwz r11, TASK_STACK - THREAD(r10) > rlwinm r12,r12,0,4,2 /* Clear SO bit in CR */ > @@ -176,6 +182,11 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_EMB_HV) > mtspr SPRN_SRR0,r11 > SYNC > RFI /* jump to handler, enable MMU */ > +#ifdef CONFIG_SMP > +1: > + lwz r5, TASK_CPU - THREAD(r10) > + RFI > +#endif > .endm > > /* To handle the additional exception priority levels on 40x and Book-E > diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile > index 06f54d947057..e147bbdc12cd 100644 > --- a/arch/powerpc/kernel/vdso32/Makefile > +++ b/arch/powerpc/kernel/vdso32/Makefile > @@ -2,9 +2,7 @@ > > # List of files in the vdso, has to be asm only for now > > -obj-vdso32-$(CONFIG_PPC64) = getcpu.o > -obj-vdso32 = sigtramp.o gettimeofday.o datapage.o cacheflush.o note.o \ > - $(obj-vdso32-y) > +obj-vdso32 = sigtramp.o gettimeofday.o datapage.o cacheflush.o note.o getcpu.o > > # Build rules > > diff --git a/arch/powerpc/kernel/vdso32/getcpu.S b/arch/powerpc/kernel/vdso32/getcpu.S > index 63e914539e1a..bde226ad904d 100644 > --- a/arch/powerpc/kernel/vdso32/getcpu.S > +++ b/arch/powerpc/kernel/vdso32/getcpu.S > @@ -17,7 +17,14 @@ > */ > V_FUNCTION_BEGIN(__kernel_getcpu) > .cfi_startproc > +#if defined(CONFIG_PPC64) > mfspr r5,SPRN_SPRG_VDSO_READ > +#elif defined(CONFIG_SMP) > + li r0, NR_MAGIC_FAST_VDSO_SYSCALL > + sc /* returns cpuid in r5, clobbers cr0 and r10-r13 */ > +#else > + li r5, 0 > +#endif > cmpwi cr0,r3,0 > cmpwi cr1,r4,0 > clrlwi r6,r5,16 > diff --git a/arch/powerpc/kernel/vdso32/vdso32.lds.S b/arch/powerpc/kernel/vdso32/vdso32.lds.S > index 099a6db14e67..663880671e20 100644 > --- a/arch/powerpc/kernel/vdso32/vdso32.lds.S > +++ b/arch/powerpc/kernel/vdso32/vdso32.lds.S > @@ -152,9 +152,7 @@ VERSION > __kernel_sync_dicache_p5; > __kernel_sigtramp32; > __kernel_sigtramp_rt32; > -#ifdef CONFIG_PPC64 > __kernel_getcpu; > -#endif > __kernel_time; > > local: *; > -- > 2.13.3
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