Hardware watchpoints on GNU/Linux AArch64

Andrew Pinski pinskia@gmail.com
Mon Jan 2 17:28:21 GMT 2023

On Mon, Jan 2, 2023 at 8:49 AM Florian Weimer via Gdb
<gdb@sourceware.org> wrote:
> Running
> gdb-12.1-10.fc38.aarch64
> kernel-6.2.0-0.rc1.20221230gitbff687b3dad6.15.fc38.aarch64
> I do not seem to be able to use hardware watchpoints.  GDB just logs
> warnings:
> warning: Unable to determine the number of hardware watchpoints available.
> warning: Unable to determine the number of hardware breakpoints available.

This shows up when the ptrace either fails or ptrace returns an
unknown value for AARCH64_DEBUG_ARCH value.
In the boot log there should be a message saying how many HW
breakpoint and watchpoints there are and what version of the debug
arch is there.
The supported values for debug arch are:
#define AARCH64_DEBUG_ARCH_V8 0x6
#define AARCH64_DEBUG_ARCH_V8_1 0x7
#define AARCH64_DEBUG_ARCH_V8_2 0x8
#define AARCH64_DEBUG_ARCH_V8_4 0x9

I don't know of any other values that can be reported, especially for
Neoverse-N1 (which should be 8_2). Neoverse-N2 is 8_4.

Andrew Pinski

> I see AArch64-specific code in GDB, so this is a bit of a surprise to
> me.
> The machine uses virtualization:
> Architecture:           aarch64
>   CPU op-mode(s):       32-bit, 64-bit
>   Byte Order:           Little Endian
> CPU(s):                 3
>   On-line CPU(s) list:  0-2
> Vendor ID:              ARM
>   BIOS Vendor ID:       QEMU
>   Model name:           Neoverse-N1
>     BIOS Model name:    virt-rhel8.6.0  CPU @ 2.0GHz
>     BIOS CPU family:    1
>     Model:              1
>     Thread(s) per core: 1
>     Core(s) per socket: 1
>     Socket(s):          3
>     Stepping:           r3p1
>     BogoMIPS:           50.00
>     Flags:              fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp
>                          asimdhp cpuid asimdrdm lrcpc dcpop asimddp ssbs
> Is this actually expected to work?  Would we have more luck on a
> bare-metal system?
> Thanks,
> Florian

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