[PATCH 0/2] Fix some arm64 perf issues

[PATCH 0/2] Fix some arm64 perf issues

[Bhupesh Sharma]

This patchset tries to address some arm64 perf issues.

While the 1st patch is a generic one and just removes a dead/left-over
declaration:
[PATCH 1/2]: hw_breakpoint: Remove unused '__register_perf_hw_breakpoint' declaration

the 2nd patch addresses 'perf stat -e' like use-cases on arm64 which
allow per-task address execution h/w breakpoints:
[PATCH 2/2]: perf/arm64: Allow per-task kernel breakpoints

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Bhupesh Sharma (2):
  hw_breakpoint: Remove unused '__register_perf_hw_breakpoint'
    declaration
  perf/arm64: Allow per-task kernel breakpoints

 arch/arm64/kernel/hw_breakpoint.c | 7 -------
 include/linux/hw_breakpoint.h     | 3 ---
 2 files changed, 10 deletions(-)

-- 
2.7.4

[PATCH 1/2] hw_breakpoint: Remove unused '__register_perf_hw_breakpoint' declaration

[Bhupesh Sharma]

commit b326e9560a28 ("hw-breakpoints: Use overflow handler instead of
the event callback") removed '__register_perf_hw_breakpoint' function
usage and replaced it with 'register_perf_hw_breakpoint' function.

Remove the left-over unused '__register_perf_hw_breakpoint' declaration
from 'hw_breakpoint.h' as well.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 include/linux/hw_breakpoint.h | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index 6058c3844a76..fe1302da8e0f 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -72,7 +72,6 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
 			    void *context);
 
 extern int register_perf_hw_breakpoint(struct perf_event *bp);
-extern int __register_perf_hw_breakpoint(struct perf_event *bp);
 extern void unregister_hw_breakpoint(struct perf_event *bp);
 extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
 
@@ -115,8 +114,6 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
 			    void *context)		{ return NULL; }
 static inline int
 register_perf_hw_breakpoint(struct perf_event *bp)	{ return -ENOSYS; }
-static inline int
-__register_perf_hw_breakpoint(struct perf_event *bp) 	{ return -ENOSYS; }
 static inline void unregister_hw_breakpoint(struct perf_event *bp)	{ }
 static inline void
 unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)	{ }
-- 
2.7.4

[PATCH 2/2] perf/arm64: Allow per-task kernel breakpoints

[Bhupesh Sharma]

commit 478fcb2cdb23 ("arm64: Debugging support") disallowed per-task
kernel breakpoints on arm64 since these would have potentially
complicated the stepping code.

However, we now have several use-cases (for e.g. perf) which require
per-task address execution h/w breakpoint to be exercised/set on arm64:

For e.g. we can set address execution h/w breakpoints, using the
format prescribed by 'perf-list' command:
mem:<addr>[/len][:access]                          [Hardware breakpoint]

Without this patch, currently 'perf stat -e' reports that per-task
address execution h/w breakpoints are 'not supported' on arm64. See
below:

$ TEST_FUNC="vfs_read"
$ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
$ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null

<not supported>;;mem:0xffff00001031dd68:x;0;100.00;;

After this patch, this use-case can be supported:

$ TEST_FUNC="vfs_read"
$ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
$ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null

5;;mem:0xfffffe0010361d20:x;912200;100.00;;

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 arch/arm64/kernel/hw_breakpoint.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index 0b727edf4104..c28f04e02845 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -562,13 +562,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
 	hw->address &= ~alignment_mask;
 	hw->ctrl.len <<= offset;
 
-	/*
-	 * Disallow per-task kernel breakpoints since these would
-	 * complicate the stepping code.
-	 */
-	if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
-		return -EINVAL;
-
 	return 0;
 }
 
-- 
2.7.4

Re: [PATCH 2/2] perf/arm64: Allow per-task kernel breakpoints

[Will Deacon]

On Tue, Feb 04, 2020 at 03:56:24AM +0530, Bhupesh Sharma wrote:
> commit 478fcb2cdb23 ("arm64: Debugging support") disallowed per-task
> kernel breakpoints on arm64 since these would have potentially
> complicated the stepping code.
> 
> However, we now have several use-cases (for e.g. perf) which require
> per-task address execution h/w breakpoint to be exercised/set on arm64:

To be honest, the perf interface to hw_breakpoint is an abomination and
I think we should remove it entirely for arm64. It's flakey, complicated,
adds code to context-switch and reduces the capabilities available to
ptrace.

> For e.g. we can set address execution h/w breakpoints, using the
> format prescribed by 'perf-list' command:
> mem:<addr>[/len][:access]                          [Hardware breakpoint]
> 
> Without this patch, currently 'perf stat -e' reports that per-task
> address execution h/w breakpoints are 'not supported' on arm64. See
> below:
> 
> $ TEST_FUNC="vfs_read"
> $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
> $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null
> 
> <not supported>;;mem:0xffff00001031dd68:x;0;100.00;;
> 
> After this patch, this use-case can be supported:
> 
> $ TEST_FUNC="vfs_read"
> $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
> $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null
> 
> 5;;mem:0xfffffe0010361d20:x;912200;100.00;;
> 
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
> ---
>  arch/arm64/kernel/hw_breakpoint.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index 0b727edf4104..c28f04e02845 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -562,13 +562,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  	hw->address &= ~alignment_mask;
>  	hw->ctrl.len <<= offset;
>  
> -	/*
> -	 * Disallow per-task kernel breakpoints since these would
> -	 * complicate the stepping code.
> -	 */
> -	if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> -		return -EINVAL;
> -

Sorry, but this is broken; the check is there for a reason, not just for
fun!

Look at how the step handler toggles the bp registers.

Will

Re: [PATCH 2/2] perf/arm64: Allow per-task kernel breakpoints

[Bhupesh Sharma]

Hi Will,

Thanks for your inputs.

On Thu, Feb 6, 2020 at 4:09 PM Will Deacon <will@kernel.org> wrote:
>
> On Tue, Feb 04, 2020 at 03:56:24AM +0530, Bhupesh Sharma wrote:
> > commit 478fcb2cdb23 ("arm64: Debugging support") disallowed per-task
> > kernel breakpoints on arm64 since these would have potentially
> > complicated the stepping code.
> >
> > However, we now have several use-cases (for e.g. perf) which require
> > per-task address execution h/w breakpoint to be exercised/set on arm64:
>
> To be honest, the perf interface to hw_breakpoint is an abomination and
> I think we should remove it entirely for arm64. It's flakey, complicated,
> adds code to context-switch and reduces the capabilities available to
> ptrace.

Sure, I agree.

> > For e.g. we can set address execution h/w breakpoints, using the
> > format prescribed by 'perf-list' command:
> > mem:<addr>[/len][:access]                          [Hardware breakpoint]
> >
> > Without this patch, currently 'perf stat -e' reports that per-task
> > address execution h/w breakpoints are 'not supported' on arm64. See
> > below:
> >
> > $ TEST_FUNC="vfs_read"
> > $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
> > $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null
> >
> > <not supported>;;mem:0xffff00001031dd68:x;0;100.00;;
> >
> > After this patch, this use-case can be supported:
> >
> > $ TEST_FUNC="vfs_read"
> > $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '`
> > $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null
> >
> > 5;;mem:0xfffffe0010361d20:x;912200;100.00;;
> >
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
> > ---
> >  arch/arm64/kernel/hw_breakpoint.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> > index 0b727edf4104..c28f04e02845 100644
> > --- a/arch/arm64/kernel/hw_breakpoint.c
> > +++ b/arch/arm64/kernel/hw_breakpoint.c
> > @@ -562,13 +562,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
> >       hw->address &= ~alignment_mask;
> >       hw->ctrl.len <<= offset;
> >
> > -     /*
> > -      * Disallow per-task kernel breakpoints since these would
> > -      * complicate the stepping code.
> > -      */
> > -     if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
> > -             return -EINVAL;
> > -
>
> Sorry, but this is broken; the check is there for a reason, not just for
> fun!
>
> Look at how the step handler toggles the bp registers.

Not sure I follow. Can you please give me some pointers. All the perf
tests I have from test-suite run fine with this chunk removed.

Thanks for your help.
Regards,
Bhupesh