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Message-ID: <20231028045813.685389-1-debug@rivosinc.com>
Date: Fri, 27 Oct 2023 21:58:00 -0700
From: Deepak Gupta <debug@...osinc.com>
To: linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
Jan Kiszka <jan.kiszka@...mens.com>,
Kieran Bingham <kbingham@...nel.org>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>
Cc: Deepak Gupta <debug@...osinc.com>,
Andrew Jones <ajones@...tanamicro.com>,
Palmer Dabbelt <palmer@...osinc.com>,
Hsieh-Tseng Shen <woodrow.shen@...ive.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Glenn Washburn <development@...icientek.com>,
Jeff Xie <xiehuan09@...il.com>
Subject: [PATCH v6] scripts/gdb: add lx_current support for riscv
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)
Signed-off-by: Deepak Gupta <debug@...osinc.com>
Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
Reviewed-by: Palmer Dabbelt <palmer@...osinc.com>
Acked-by: Palmer Dabbelt <palmer@...osinc.com>
Tested-by: Hsieh-Tseng Shen <woodrow.shen@...ive.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
v5 --> v6:
- rebased on 6.6-rc5. dropped changes in utils.py as they're 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..cba589e5b57d 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")
--
2.42.0
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