[PATCH v2 1/7] gdb/aarch64: Add POR_EL0 register support for FEAT_S1POE

[<srinath.parvathaneni@arm.com>]

From: Srinath Parvathaneni <srinath.parvathaneni@arm.com>

Add support for the FEAT_S1POE POR_EL0 register on AArch64.

This patch adds POR_EL0 to the AArch64 register set and reads/writes it
using the NT_ARM_POE ptrace regset.

With this change, POR_EL0 is available through:
*  info registers
*  info registers por_el0
*  p $por_el0
*  p/x $por_el0
*  set $por_el0 = <value>
---
 gdb/Makefile.in              |  2 ++
 gdb/aarch64-linux-nat.c      | 66 ++++++++++++++++++++++++++++++++++++
 gdb/aarch64-linux-tdep.c     |  1 +
 gdb/aarch64-tdep.c           | 18 ++++++++++
 gdb/aarch64-tdep.h           | 11 +++++-
 gdb/arch/aarch64-poe-linux.h | 29 ++++++++++++++++
 gdb/arch/aarch64.c           |  4 +++
 gdb/arch/aarch64.h           |  7 +++-
 gdb/features/Makefile        |  1 +
 gdb/features/aarch64-poe.c   | 14 ++++++++
 gdb/features/aarch64-poe.xml | 12 +++++++
 gdb/nat/aarch64-poe-linux.h  | 28 +++++++++++++++
 include/elf/common.h         |  2 ++
 13 files changed, 193 insertions(+), 2 deletions(-)
 create mode 100644 gdb/arch/aarch64-poe-linux.h
 create mode 100644 gdb/features/aarch64-poe.c
 create mode 100644 gdb/features/aarch64-poe.xml
 create mode 100644 gdb/nat/aarch64-poe-linux.h

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 57f384170ab..3dc9cd07dcb 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -1292,6 +1292,7 @@ HFILES_NO_SRCDIR = \
 	arch/aarch32.h \
 	arch/aarch64-fpmr-linux.h \
 	arch/aarch64-gcs-linux.h \
+	arch/aarch64-poe-linux.h \
 	arch/aarch64.h \
 	arch/aarch64-insn.h \
 	arch/aarch64-mte.h \
@@ -1540,6 +1541,7 @@ HFILES_NO_SRCDIR = \
 	namespace.h \
 	nat/aarch64-fpmr-linux.h \
 	nat/aarch64-gcs-linux.h \
+	nat/aarch64-poe-linux.h \
 	nat/aarch64-hw-point.h \
 	nat/aarch64-linux.h \
 	nat/aarch64-linux-hw-point.h \
diff --git a/gdb/aarch64-linux-nat.c b/gdb/aarch64-linux-nat.c
index 52ace4aab41..54265920d69 100644
--- a/gdb/aarch64-linux-nat.c
+++ b/gdb/aarch64-linux-nat.c
@@ -34,6 +34,7 @@
 #include "arch/arm.h"
 #include "nat/aarch64-fpmr-linux.h"
 #include "nat/aarch64-gcs-linux.h"
+#include "nat/aarch64-poe-linux.h"
 #include "nat/aarch64-linux.h"
 #include "nat/aarch64-linux-hw-point.h"
 #include "nat/aarch64-mte-linux-ptrace.h"
@@ -569,6 +570,54 @@ fetch_gcsregs_from_thread (regcache *regcache)
 			&user_gcs.features_locked);
 }
 
+/* Fill GDB's register array with the POE register value from the current
+   thread.  */
+
+static void
+fetch_poeregs_from_thread (regcache *regcache)
+{
+  aarch64_gdbarch_tdep *tdep
+    = gdbarch_tdep<aarch64_gdbarch_tdep> (regcache->arch ());
+
+  gdb_assert (tdep->has_poe ());
+
+  uint64_t user_poe;
+  iovec iovec;
+
+  iovec.iov_base = &user_poe;
+  iovec.iov_len = sizeof (user_poe);
+
+  int tid = get_ptrace_pid (regcache->ptid ());
+  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_POE, &iovec) != 0)
+    perror_with_name (_("Unable to fetch POE register"));
+
+  regcache->raw_supply (tdep->poe_regnum, &user_poe);
+}
+
+/* Store the NT_ARM_POE register contents from GDB's REGCACHE to the
+    thread associated with REGCACHE.  */
+
+static void
+store_poeregs_to_thread (struct regcache *regcache)
+{
+  aarch64_gdbarch_tdep *tdep
+    = gdbarch_tdep<aarch64_gdbarch_tdep> (regcache->arch ());
+
+  gdb_assert (tdep->has_poe ());
+
+  int tid = regcache->ptid ().lwp ();
+
+  iovec iovec;
+  uint64_t user_poe;
+  iovec.iov_base = &user_poe;
+  iovec.iov_len = sizeof (user_poe);
+
+  regcache->raw_collect (tdep->poe_regnum, &user_poe);
+
+  if (ptrace (PTRACE_SETREGSET, tid, NT_ARM_POE, &iovec) != 0)
+    perror_with_name (_("Unable to store POE register"));
+}
+
 /* Store to the current thread the valid GCS register set in the GDB's
    register array.  */
 
@@ -683,6 +732,9 @@ aarch64_fetch_registers (struct regcache *regcache, int regno)
       if (tdep->has_gcs_linux ())
 	fetch_gcsregs_from_thread (regcache);
 
+      if (tdep->has_poe ())
+	fetch_poeregs_from_thread (regcache);
+
       if (tdep->has_fpmr ())
 	fetch_fpmr_from_thread (regcache);
     }
@@ -722,6 +774,10 @@ aarch64_fetch_registers (struct regcache *regcache, int regno)
 	   && (regno == tdep->gcs_reg_base || regno == tdep->gcs_linux_reg_base
 	       || regno == tdep->gcs_linux_reg_base + 1))
     fetch_gcsregs_from_thread (regcache);
+  /* POE register?  */
+  else if (tdep->has_poe () && (regno == tdep->poe_regnum))
+    fetch_poeregs_from_thread (regcache);
+
   /* FPMR?  */
   else if (tdep->has_fpmr () && (regno == tdep->fpmr_regnum))
     fetch_fpmr_from_thread (regcache);
@@ -802,6 +858,9 @@ aarch64_store_registers (struct regcache *regcache, int regno)
 
       if (tdep->has_fpmr ())
 	store_fpmr_to_thread (regcache);
+
+      if (tdep->has_poe ())
+	store_poeregs_to_thread (regcache);
     }
   /* General purpose register?  */
   else if (regno < AARCH64_V0_REGNUM)
@@ -837,6 +896,10 @@ aarch64_store_registers (struct regcache *regcache, int regno)
   else if (tdep->has_fpmr () && regno == tdep->fpmr_regnum)
     store_fpmr_to_thread (regcache);
 
+  /* POE register?  */
+  else if (tdep->has_poe () && regno == tdep->poe_regnum)
+    store_poeregs_to_thread (regcache);
+
   /* PAuth registers are read-only.  */
 }
 
@@ -1024,6 +1087,9 @@ aarch64_linux_nat_target::read_description ()
   /* Check for FPMR.  */
   features.fpmr = hwcap2 & HWCAP2_FPMR;
 
+  /* Check for POE support.  */
+  features.poe = hwcap2 & HWCAP2_POE;
+
   return aarch64_read_description (features);
 }
 
diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
index f11eccc1bc1..4a6bc3158c8 100644
--- a/gdb/aarch64-linux-tdep.c
+++ b/gdb/aarch64-linux-tdep.c
@@ -53,6 +53,7 @@
 
 #include "arch/aarch64-fpmr-linux.h"
 #include "arch/aarch64-gcs-linux.h"
+#include "arch/aarch64-poe-linux.h"
 #include "arch/aarch64-mte.h"
 #include "arch/aarch64-mte-linux.h"
 #include "arch/aarch64-pauth-linux.h"
diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 886e7ce1e7b..f1e1a9c7a19 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -164,6 +164,11 @@ static const char *const aarch64_gcs_register_names[] = {
   "gcspr"
 };
 
+static const char *const aarch64_poe_register_names[] = {
+  /* Permission Overlay Extension Register.  */
+  "por_el0"
+};
+
 static const char *const aarch64_gcs_linux_register_names[] = {
   /* Field in struct user_gcs.  */
   "gcs_features_enabled",
@@ -4556,6 +4561,19 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
 					  fpmr_regnum, "fpmr");
     }
 
+  int poe_regnum = -1;
+  const struct tdesc_feature *feature_poe
+      = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.poe");
+  if (feature_poe != nullptr)
+    {
+      poe_regnum = num_regs;
+      for (i = 0; i < ARRAY_SIZE (aarch64_poe_register_names); i++)
+	valid_p &= tdesc_numbered_register (feature_poe, tdesc_data.get (),
+					    poe_regnum + i,
+					    aarch64_poe_register_names[i]);
+      num_regs++;
+    }
+
   int first_sme_regnum = -1;
   int first_sme2_regnum = -1;
   int first_sme_pseudo_regnum = -1;
diff --git a/gdb/aarch64-tdep.h b/gdb/aarch64-tdep.h
index dff05a08f84..8029725b145 100644
--- a/gdb/aarch64-tdep.h
+++ b/gdb/aarch64-tdep.h
@@ -208,6 +208,16 @@ struct aarch64_gdbarch_tdep : gdbarch_tdep_base
     return gcs_linux_reg_base != -1;
   }
 
+  /* POE register.  This is -1 if no POE feature is available.  */
+  int poe_regnum = -1;
+
+  /* Returns true if the target supports the Linux POE feature.  */
+  bool
+  has_poe () const
+  {
+    return poe_regnum != -1;
+  }
+
   /* First FPMR register.  This is -1 if FPMR is not supported.  */
   int fpmr_regnum = -1;
 
@@ -224,7 +234,6 @@ aarch64_features_from_target_desc (const struct target_desc *tdesc);
 
 extern int aarch64_process_record (struct gdbarch *gdbarch,
 			       struct regcache *regcache, CORE_ADDR addr);
-
 displaced_step_copy_insn_closure_up
   aarch64_displaced_step_copy_insn (struct gdbarch *gdbarch,
 				    CORE_ADDR from, CORE_ADDR to,
diff --git a/gdb/arch/aarch64-poe-linux.h b/gdb/arch/aarch64-poe-linux.h
new file mode 100644
index 00000000000..d6e4a001cbf
--- /dev/null
+++ b/gdb/arch/aarch64-poe-linux.h
@@ -0,0 +1,29 @@
+/* Common Linux target-dependent definitions for AArch64 POE
+
+   Copyright (C) 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_ARCH_AARCH64_POE_LINUX_H
+#define GDB_ARCH_AARCH64_POE_LINUX_H
+
+/* Feature check for Permission Overlay Extension.  */
+#define AARCH64_HWCAP2_POE (1ULL << 63)
+
+/* Data or instruction abort caused by Protection Key Violation.  */
+#define AARCH64_SEGV_PKUERR 4
+
+#endif /* GDB_ARCH_AARCH64_POE_LINUX_H */
diff --git a/gdb/arch/aarch64.c b/gdb/arch/aarch64.c
index 2401a325b7b..a567938d06d 100644
--- a/gdb/arch/aarch64.c
+++ b/gdb/arch/aarch64.c
@@ -28,6 +28,7 @@
 #include "../features/aarch64-sme2.c"
 #include "../features/aarch64-tls.c"
 #include "../features/aarch64-gcs.c"
+#include "../features/aarch64-poe.c"
 #include "../features/aarch64-gcs-linux.c"
 
 /* See arch/aarch64.h.  */
@@ -77,6 +78,9 @@ aarch64_create_target_description (const aarch64_features &features)
   if (features.fpmr)
     regnum = create_feature_aarch64_fpmr (tdesc.get (), regnum);
 
+  if (features.poe)
+    regnum = create_feature_aarch64_poe (tdesc.get (), regnum);
+
   return tdesc;
 }
 
diff --git a/gdb/arch/aarch64.h b/gdb/arch/aarch64.h
index cf7318cbe74..4e4b50f729e 100644
--- a/gdb/arch/aarch64.h
+++ b/gdb/arch/aarch64.h
@@ -35,6 +35,7 @@ struct aarch64_features
   bool pauth = false;
   bool mte = false;
   bool fpmr = false;
+  bool poe = false;
 
   /* A positive TLS value indicates the number of TLS registers available.  */
   uint8_t tls = 0;
@@ -70,7 +71,8 @@ inline bool operator==(const aarch64_features &lhs, const aarch64_features &rhs)
     && lhs.sme2 == rhs.sme2
     && lhs.gcs == rhs.gcs
     && lhs.gcs_linux == rhs.gcs_linux
-    && lhs.fpmr == rhs.fpmr;
+    && lhs.fpmr == rhs.fpmr
+    && lhs.poe == rhs.poe;
 }
 
 namespace std
@@ -103,6 +105,9 @@ namespace std
 
       /* FPMR feature.  */
       h = h << 1 | features.fpmr;
+
+      /* POE feature.  */
+      h = h << 1 | features.poe;
       return h;
     }
   };
diff --git a/gdb/features/Makefile b/gdb/features/Makefile
index 161a7453d66..cc77e5bb652 100644
--- a/gdb/features/Makefile
+++ b/gdb/features/Makefile
@@ -207,6 +207,7 @@ FEATURE_XMLFILES = aarch64-core.xml \
 	aarch64-pauth.xml \
 	aarch64-mte.xml \
 	aarch64-gcs.xml \
+	aarch64-poe.xml \
 	aarch64-gcs-linux.xml \
 	arc/v1-core.xml \
 	arc/v1-aux.xml \
diff --git a/gdb/features/aarch64-poe.c b/gdb/features/aarch64-poe.c
new file mode 100644
index 00000000000..e964a625173
--- /dev/null
+++ b/gdb/features/aarch64-poe.c
@@ -0,0 +1,14 @@
+/* THIS FILE IS GENERATED.  -*- buffer-read-only: t -*- vi:set ro:
+  Original: aarch64-poe.xml */
+
+#include "gdbsupport/tdesc.h"
+
+static int
+create_feature_aarch64_poe (struct target_desc *result, long regnum)
+{
+  struct tdesc_feature *feature;
+
+  feature = tdesc_create_feature (result, "org.gnu.gdb.aarch64.poe");
+  tdesc_create_reg (feature, "por_el0", regnum++, 1, "system", 64, "uint64");
+  return regnum;
+}
diff --git a/gdb/features/aarch64-poe.xml b/gdb/features/aarch64-poe.xml
new file mode 100644
index 00000000000..43ee09e01d6
--- /dev/null
+++ b/gdb/features/aarch64-poe.xml
@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<!-- Copyright (C) 2026 Free Software Foundation, Inc.
+
+     Copying and distribution of this file, with or without modification,
+     are permitted in any medium without royalty provided the copyright
+     notice and this notice are preserved.  -->
+
+<!DOCTYPE feature SYSTEM "gdb-target.dtd">
+<feature name="org.gnu.gdb.aarch64.poe">
+
+  <reg name="por_el0" bitsize="64" type="uint64" group="system"/>
+</feature>
diff --git a/gdb/nat/aarch64-poe-linux.h b/gdb/nat/aarch64-poe-linux.h
new file mode 100644
index 00000000000..3d858d3ff92
--- /dev/null
+++ b/gdb/nat/aarch64-poe-linux.h
@@ -0,0 +1,28 @@
+/* Common native Linux definitions for AArch64 Permission Overlay Extension.
+
+   Copyright (C) 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_NAT_AARCH64_POE_LINUX_H
+#define GDB_NAT_AARCH64_POE_LINUX_H
+
+/* Feature check for Permission Overlay Extension.  */
+#ifndef HWCAP2_POE
+#define HWCAP2_POE (1ULL << 63)
+#endif /* HWCAP2_POE */
+
+#endif /* GDB_NAT_AARCH64_POE_LINUX_H */
diff --git a/include/elf/common.h b/include/elf/common.h
index 1ae68221a89..f811b8daeb5 100644
--- a/include/elf/common.h
+++ b/include/elf/common.h
@@ -760,6 +760,8 @@
 					/*   Note: name must be "LINUX".  */
 #define NT_ARM_FPMR     0x40e           /* AArch64 FPMR.  */
 					/*   Note: name must be "LINUX".  */
+#define NT_ARM_POE	0x40f		/* AArch64 POE register.  */
+					/*   Note: name must be "LINUX".  */
 #define NT_ARM_GCS	0x410		/* AArch64 Guarded Control Stack
 					   registers.  */
 					/*   Note  name must be "LINUX".  */
-- 
2.43.0