> -----Original Message----- > From: Jan Kiszka [mailto:jan.kiszka@xxxxxxxxxxx] > Sent: Tuesday, February 23, 2021 8:27 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; > kieran.bingham@xxxxxxxxxxxxxxxx; corbet@xxxxxxx; linux-doc@xxxxxxxxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx > Subject: Re: [PATCH] scripts/gdb: document lx_current is only supported by x86 > > On 22.02.21 22:18, Song Bao Hua (Barry Song) wrote: > > > > > >> -----Original Message----- > >> From: Kieran Bingham [mailto:kieran.bingham@xxxxxxxxxxxxxxxx] > >> Sent: Tuesday, February 23, 2021 12:06 AM > >> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; corbet@xxxxxxx; > >> linux-doc@xxxxxxxxxxxxxxx; jan.kiszka@xxxxxxxxxxx > >> Cc: linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx > >> Subject: Re: [PATCH] scripts/gdb: document lx_current is only supported by > x86 > >> > >> Hi Barry > >> > >> On 21/02/2021 21:35, Barry Song wrote: > >>> lx_current depends on the per_cpu current_task which exists on x86 only: > >>> > >>> arch$ git grep current_task | grep -i per_cpu > >>> x86/include/asm/current.h:DECLARE_PER_CPU(struct task_struct *, > >> current_task); > >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, > current_task) > >> ____cacheline_aligned = > >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task); > >>> x86/kernel/cpu/common.c:DEFINE_PER_CPU(struct task_struct *, > current_task) > >> = &init_task; > >>> x86/kernel/cpu/common.c:EXPORT_PER_CPU_SYMBOL(current_task); > >>> x86/kernel/smpboot.c: per_cpu(current_task, cpu) = idle; > >>> > >>> On other architectures, lx_current() will lead to a python exception: > >>> (gdb) p $lx_current().pid > >>> Python Exception <class 'gdb.error'> No symbol "current_task" in current > >> context.: > >>> Error occurred in Python: No symbol "current_task" in current context. > >>> > >>> To avoid more people struggling and wasting time in other architectures, > >>> document it. > >>> > >>> Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> > >>> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx> > >>> --- > >>> Documentation/dev-tools/gdb-kernel-debugging.rst | 2 +- > >>> scripts/gdb/linux/cpus.py | 10 ++++++++-- > >>> 2 files changed, 9 insertions(+), 3 deletions(-) > >>> > >>> diff --git a/Documentation/dev-tools/gdb-kernel-debugging.rst > >> b/Documentation/dev-tools/gdb-kernel-debugging.rst > >>> index 4756f6b3a04e..1586901b683c 100644 > >>> --- a/Documentation/dev-tools/gdb-kernel-debugging.rst > >>> +++ b/Documentation/dev-tools/gdb-kernel-debugging.rst > >>> @@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb helpers > >>> [ 0.000000] BIOS-e820: [mem > 0x000000000009fc00-0x000000000009ffff] > >> reserved > >>> .... > >>> > >>> -- Examine fields of the current task struct:: > >>> +- Examine fields of the current task struct(supported by x86 only):: > >>> > >>> (gdb) p $lx_current().pid > >>> $1 = 4998 > >>> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py > >>> index 008e62f3190d..f382762509d3 100644 > >>> --- a/scripts/gdb/linux/cpus.py > >>> +++ b/scripts/gdb/linux/cpus.py > >>> @@ -156,6 +156,13 @@ Note that VAR has to be quoted as string.""" > >>> > >>> PerCpu() > >>> > >>> +def get_current_task(cpu): > >>> + if utils.is_target_arch("x86"): > >>> + var_ptr = gdb.parse_and_eval("¤t_task") > >>> + return per_cpu(var_ptr, cpu).dereference() > >>> + else: > >>> + raise gdb.GdbError("Sorry, obtaining the current task is not yet > " > >>> + "supported with this arch") > >> > >> I've wondered in the past how we should handle the architecture specific > >> layers. > >> > >> Perhaps we need to have an interface of functionality to implement on > >> each architecture so that we can create a per-arch set of helpers. > >> > >> or break it up into arch specific subdirs at least... > >> > >> > >>> class LxCurrentFunc(gdb.Function): > >>> """Return current task. > >>> @@ -167,8 +174,7 @@ number. If CPU is omitted, the CPU of the current context > >> is used.""" > >>> super(LxCurrentFunc, self).__init__("lx_current") > >>> > >>> def invoke(self, cpu=-1): > >>> - var_ptr = gdb.parse_and_eval("¤t_task") > >>> - return per_cpu(var_ptr, cpu).dereference() > >>> + return get_current_task(cpu) > >>> > >> > >> And then perhaps we simply shouldn't even expose commands which can not > >> be supported on those architectures? > > > > I feel it is better to tell users this function is not supported on its arch > > than simply hiding the function. > > > > If we hide it, users still have many chances to try it as they have got > > information of lx_current from google or somewhere. > > They will try, if it turns out the lx_current is not in the list and an > > error like "invalid data type for function to be called", they will > > probably suspect their gdb/python environment is not set up correctly, > > and continue to waste time in checking their environment. > > Finally they figure out this function is not supported by its arch so it is > > not exposed. But they have wasted a couple of hours before knowing that. > > > > It seems it is more friendly to directly tell users this is not supported > > on its arch explicitly and clearly than reporting a "invalid data type > > for function to be called. > > > >> > >> Is it easy to disable this command if it's not supportable on the > >> architecture? > >> > > > > TBH, I'm not a python expert. I don't know how to do that in an elegant > > way :-) on the other hand, it seems lx_current isn’t a command like > > lx-dmesg. Lx_current is actually similar with lx_per_cpu, we use gdb's > > print(p) command to show its content. > > > >> Presumably you are working on non-x86, have you investigated adding this > >> support for your architecture (arm/arm64?)? > > > > Yes. I've thought about it. But It would be quite trivial to bring up > > this function on arm64. > > > > arch/arm64/include/asm/current.h: > > static __always_inline struct task_struct *get_current(void) > > { > > unsigned long sp_el0; > > > > asm ("mrs %0, sp_el0" : "=r" (sp_el0)); > > > > return (struct task_struct *)sp_el0; > > } > > > > We have to read a special register named sp_el0 and convert it to > > task_struct while we are running in kernel mode. In gdb I can do > > it by: > > (gdb)p/x $SP_EL0 > > $20 = 0xffffffc011492400 > > (gdb)p ((struct task_struct *0xffffffc011492400))->pid > > $21 = 0 > > > > What is more complex is that if we are running in user mode(EL0), this > > register doesn't describe current task any more. so we have to > > differentiate the modes of processor and make sure it only returns > > current task while we are running in EL1(processor's kernel mode). > > Is all information needed for this available via gdb? I can't read current EL level from gdb as CurrentEL is not readable from EL0 as shown on the ARMv8 manual C5.2.1: "CurrentEL, Current Exception Level" section "Accessibility". Trying to run it in Linux userland raises SIGILL. But a workaround I can do is that while running in kernel, SP_EL0 is a value like 0xffffxxxx xxxxxxxx; otherwise, it would be a value like 0x0000xxxx xxxxxxxx. So I could actually implement lx_current on arm64 by: p/x $SP_EL0 if value > 0xffff00000000000 task_struct_addr = value else userspace, no current the problem is that I don't know how to read the register and transfer address into task_struct in gdb/scripts. Would you like to share some example code if you have? > > > > >> > >> The fact you have run the command implies it would be useful for you ? > >> > > > > Yes. I think it is a common requirement to get current task. lx_current > > convenience function can help everyone. Since there is a document saying > > this command exists, everyone using scripts/gdb would like to try it > > I guess. > > > > The simplest way would be adding current_task per_cpu variable for other > > arch, but I believe hardly arch maintainers will accept it as its only > > benefit is bringing up the lx_current. So 99.9% no maintainer wants it. > > > > Thus, for the time being, I moved to just stop people from wasting time > > like what I had done with a couple of hours. > > > > I agree with the warning, also as potential motivation to add support > for other archs. > Yep. > Jan > > -- > Siemens AG, T RDA IOT > Corporate Competence Center Embedded Linux Thanks Barry
