Re: [PATCH] ARC: perf: Accommodate big-endian CPU

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Nov 27, 2019 at 11:01:23AM +0300, Alexey Brodkin wrote:
> 8-letter strings representing ARC perf events are stores in two
> 32-bit registers as ASCII characters like that: "IJMP", "IALL", "IJMPTAK" etc.
> 
> And the same order of bytes in the word is used regardless CPU endianness.
> 
> Which means in case of big-endian CPU core we need to swap bytes to get
> the same order as if it was on little-endian CPU.
> 
> Otherwise we're seeing the following error message on boot:
> ------------------------->8----------------------
> ARC perf        : 8 counters (32 bits), 40 conditions, [overflow IRQ support]
> sysfs: cannot create duplicate filename '/devices/arc_pct/events/pmji'
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.2.18 #3
> Stack Trace:
>   arc_unwind_core+0xd4/0xfc
>   dump_stack+0x64/0x80
>   sysfs_warn_dup+0x46/0x58
>   sysfs_add_file_mode_ns+0xb2/0x168
>   create_files+0x70/0x2a0
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 1 at kernel/events/core.c:12144 perf_event_sysfs_init+0x70/0xa0
> Failed to register pmu: arc_pct, reason -17
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.2.18 #3
> Stack Trace:
>   arc_unwind_core+0xd4/0xfc
>   dump_stack+0x64/0x80
>   __warn+0x9c/0xd4
>   warn_slowpath_fmt+0x22/0x2c
>   perf_event_sysfs_init+0x70/0xa0
> ---[ end trace a75fb9a9837bd1ec ]---
> ------------------------->8----------------------
> 
> What happens here we're trying to register more than one raw perf event
> with the same name "PMJI". Why? Because ARC perf events are 4 to 8 letters
> and encoded into two 32-bit words. In this particular case we deal with 2
> events:
>  * "IJMP____" which counts all jump & branch instructions
>  * "IJMPC___" which counts only conditional jumps & branches
> 
> Those strings are split in two 32-bit words this way "IJMP" + "____" &
> "IJMP" + "C___" correspondingly. Now if we read them swapped due to CPU core
> being big-endian then we read "PMJI" + "____" & "PMJI" + "___C".
> 
> And since we interpret read array of ASCII letters as a null-terminated string
> on big-endian CPU we end up with 2 events of the same name "PMJI".
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> Cc: stable@vger.kernel.org
> ---
> 
> Greg, Sasha, this is the same patch as
> commit 5effc09c4907 ("ARC: perf: Accommodate big-endian CPU")
> but fine-tuned to be applicable to kernels 4.19 and older.

Thanks, now queued up.

greg k-h

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