Re: Hardware watchpoints on GNU/Linux AArch64

Andrew Pinski via Gdb
2023-01-02T18:11:22Z

On Mon, Jan 2, 2023 at 9:39 AM Andrew Pinski <pinskia@gmail.com> wrote:
>
> 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.

Looking KVM sources (arch/arm64/kvm/sys_regs.c) inside the kernel we see:
/* Limit debug to ARMv8.0 */
val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_DebugVer);
val |=
FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_EL1_DebugVer), 6);
Which was added by 94893fc9ad8cd for 5.12 unless this changed post Linux 6.0 .
So a value of 6 should be good to go.

Thanks,
Andrew Pinski

>
> 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
> > >