Re: [PATCH v5] scripts/gdb: add lx_current support for riscv

[Deepak Gupta]

On Wed, Nov 16, 2022 at 09:16:49AM +0100, Andrew Jones wrote:
> On Tue, Nov 15, 2022 at 02:10:51PM -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_sscratch CSR.
> >
> > This patch assumes "tp" is pointing to task_struct. If value in
> > csr_sscratch is numerically greater than "tp" then it assumes csr_sscratch
> > is correct address of current task_struct. This logic holds when
> >    - hart is in user space, "tp" will be less than csr_sscratch.
> >    - hart is in kernel space but not in trap handler, "tp" will be more
> >      than csr_sscratch (csr_sscratch being equal to 0).
> >    - hart is executing trap handler
> >        - "tp" is still pointing to user mode but csr_sscratch contains
> >           ptr to task_struct. Thus numerically higher.
> >        - "tp" is  pointing to task_struct but csr_sscratch now contains
> >           either 0 or numerically smaller value (transiently holds
> >           user mode tp)
> >
> > Patch also adds new cached type "ulong" in scripts/gdb/linux/utils.py
> >
> > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> >
> > ---
> > Since patch has changed a little bit from v1 and I didn't include
> > changelog earlier, here it is.
> >
> > v1 --> v2:
> >  - added logic to locate task_struct irrespective of priv
> >  - made locating task_struct agnostic to bitness(32 vs 64).
> >  - added caching of ulong type in scripts/gdb/linux/utils.py
> >  - added more descriptive commit message
> >
> > v2 --> v3:
> >  - amended commit message and source line to fit column width
> >
> > v3 --> v4:
> >  - amended commit message and remove whitespace in source
> >  - added Reviewed-by for reviewers
> >
> > v4 --> v5:
> >  - changing the order of changelog and sign off/review tags in commit
> > ---
> > ---
>
> Everything looks good, but you've got extra ---'s here. They don't hurt,
> but if you're still ironing out your workflow you may want to keep in
> mind that you don't need them. You only need one, which goes above the
> changelog.
>
> Thanks,
> drew

Noted. Thanks.

> >  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..14c22f82449b 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.
> > +         # and tp is used by user mode. So when scratch register holds larger value
> > +         # (negative address as ulong is larger value) than tp, then use scratch register.
> > +         if (scratch_reg.cast(utils.get_ulong_type()) > current_tp.cast(utils.get_ulong_type())):
> > +             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