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

Deepak Gupta
2023-10-26T23:41:32Z

On Wed, Oct 4, 2023 at 10:29 PM Hsieh-Tseng Shen
<woodrow.shen@sifive.com> wrote:
>
> On Mon, Jan 02, 2023 at 10:09:01AM +0100, Jan Kiszka wrote:
> > On 15.11.22 23:10, 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@rivosinc.com>
> > > Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> > >
> > > ---
> > > 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
> > > ---
> > > ---
> > > 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)
> >
> > Why not if-else for the assignment here?
> >
> > > +
> > > + 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)
> >
> > Looks good to me otherwise.
> >
> > Jan
> >
> > --
> > Siemens AG, Technology
> > Competence Center Embedded Linux
>
> This patch had been pending for quite a while, and not sure if it's
> still acceptable to be merged, but from my testing it's working fine
> for me. Nevertheless, the v5 patch now has conflict with the current
> master, so I've slightly modified for reference. It would be helpful
> if Deepak can send v6 later on.

Thanks for testing it out.
I just sent out a v6 based on v6.6-rc5.

-Deepak

>
> Tested-by: Hsieh-Tseng Shen <woodrow.shen@sifive.com>
>
> v5 --> v6:
> - dropped cache type "ulong" in scripts/gdb/linux/utils.py as it
> already exists in the current upstream
> ---
> scripts/gdb/linux/cpus.py | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
> index 255dc18cb9da..f8325cab5f1b 100644
> --- a/scripts/gdb/linux/cpus.py
> +++ b/scripts/gdb/linux/cpus.py
> @@ -179,6 +179,21 @@ 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")
> --
> 2.31.1
>
> Thank you