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Date:   Wed, 16 Nov 2022 14:24:48 -0800
From:   Deepak Gupta <debug@rivosinc.com>
To:     Andrew Jones <ajones@ventanamicro.com>
Cc:     conor.dooley@microchip.com, aou@eecs.berkeley.edu,
        jan.kiszka@siemens.com, kbingham@kernel.org,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        palmer@dabbelt.com, paul.walmsley@sifive.com
Subject: Re: [PATCH v5] scripts/gdb: add lx_current support for riscv
Message-ID: <20221116222448.GA2103144@debug.ba.rivosinc.com>
References: <d65bddf3-c58a-1420-bff9-8333c10edb56@microchip.com>
 <20221115221051.1871569-1-debug@rivosinc.com>
 <20221116081649.yq7cy7isxj3nmzr3@kamzik>
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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
>>