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

Deepak Gupta
2022-11-16T22:25:16Z

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@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
>> ---
>> ---
>
>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
>>