Re: Hardware watchpoints on GNU/Linux AArch64

[Andrew Pinski <pinskia@gmail.com>]

On Mon, Jan 2, 2023 at 9:28 AM Andrew Pinski <pinskia@gmail.com> wrote:
>
> 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.

The kernel fills in that field with ptrace via:
u8 debug_monitors_arch(void)
{
        return cpuid_feature_extract_unsigned_field(read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1),
                                                ID_AA64DFR0_EL1_DebugVer_SHIFT);
}

So either kvm/qemu is setting that EL1 (system) register incorrectly
or the KVM Invocation didn't turn on HW breakpoints. Or KVM does not
support HW breakpoints yet.

Thanks,
Andrew


>
> Thanks,
> 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
> >