Re: [PATCH v2] scripts/gdb: add lx_current support for riscv

Andrew Jones
2022-11-15T06:46:17Z

On Mon, Nov 14, 2022 at 05:29:17PM -0800, Deepak Gupta wrote:
> csr_sscratch CSR holds current task_struct address when hart is in user space.
> trap handler on entry spills csr_sscratch into "tp" (x2) register and zeroes out
> csr_sscratch CSR. trap handler on exit reloads "tp" with expected user mode value
> and place current task_struct address again in csr_scratch CSR.
>
> This patch assumes "tp" is pointing to task_struct. If value in csr_scratch is numerically
> greater than "tp" then it assumes csr_scratch is correct address of current task_struct.
> This logic holds when
> - hart is in user space, "tp" will be less than csr_scratch.
> - hart is in kernel space but not in trap handler, "tp" will be more than csr_scratch.
> - hart is executing trap handler
> - "tp" is still pointing to user mode but csr_scratch contains ptr to task_struct.
> numerically higher.
> - "tp" is now pointing to task_struct but csr_scratch now contains either 0 or numerically
> smaller value.

When is (csr_scratch != 0 && csr_scratch < tp)? IIUC, it should always be
zero or >= tp.

>
> Patch also adds new cached type "ulong" in scripts/gdb/linux/utils.py

Please wrap commit message lines at ~74.

>
> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
> ---
> scripts/gdb/linux/cpus.py | 15 +++++++++++++++
> scripts/gdb/linux/utils.py | 5 +++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
> index 15fc4626d236..fd818d7896ce 100644
> --- a/scripts/gdb/linux/cpus.py
> +++ b/scripts/gdb/linux/cpus.py
> @@ -173,6 +173,21 @@ def get_current_task(cpu):
> else:
> raise gdb.GdbError("Sorry, obtaining the current task is not allowed "
> "while running in userspace(EL0)")
> + elif utils.is_target_arch("riscv"):
> + current_tp = gdb.parse_and_eval("$tp")
> + scratch_reg = gdb.parse_and_eval("$sscratch")
> +
> + # by default tp points to current task
> + current_task = current_tp.cast(task_ptr_type)
> +
> + # scratch register is set 0 in trap handler after entering kernel.
> + # When hart is in user mode, scratch register is pointing to task_struct.
> + # So when scratch register holds higher value (negative address as ulong is bigger value) than tp,

Please wrap source lines at 100.

> + # then use scratch register
> + if (scratch_reg.cast(utils.get_ulong_type()) > current_tp.cast(utils.get_ulong_type())):
^ extra space here

> + current_task = scratch_reg.cast(task_ptr_type)
> +
> + return current_task.dereference()
> else:
> raise gdb.GdbError("Sorry, obtaining the current task is not yet "
> "supported with this arch")
> diff --git a/scripts/gdb/linux/utils.py b/scripts/gdb/linux/utils.py
> index 1553f68716cc..ddaf3089170d 100644
> --- a/scripts/gdb/linux/utils.py
> +++ b/scripts/gdb/linux/utils.py
> @@ -35,12 +35,17 @@ class CachedType:
>
>
> long_type = CachedType("long")
> +ulong_type = CachedType("ulong")
> atomic_long_type = CachedType("atomic_long_t")
>
> def get_long_type():
> global long_type
> return long_type.get_type()
>
> +def get_ulong_type():
> + global ulong_type
> + return ulong_type.get_type()
> +
> def offset_of(typeobj, field):
> element = gdb.Value(0).cast(typeobj)
> return int(str(element[field].address).split()[0], 16)
> --
> 2.25.1
>

Besides the line wrapping and the commit message clarification this looks
good to me.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

Thanks,
drew