[RFC] gdb/riscv: Add read_description method for riscv_linux_nat_target

[Andrew Burgess]

Adds riscv_linux_nat_target::read_description method to find a
suitable target description for the native linux target we are running
on.

Currently this will supply a suitably sized set of x-registers, and
will probe the kernel to see if the f-registers are readable. If they
are readable then we currently assume that the f-registers are the
same size as the x-registers as I don't know of a good way to probe
the f-register length. This will obviously need fixing in future.

As of Linux 4.19 there is no ptrace support for reading the
f-registers, this should appear in 4.20, so right now we only return
target descriptions without f-registers.

gdb/ChangeLog:

* riscv-linux-nat.c: Add 'inferior.h' and 'target-descriptions.h'
header files.
(riscv_linux_nat_target::read_description): New method.
---
gdb/ChangeLog | 6 ++++++
gdb/riscv-linux-nat.c | 38 ++++++++++++++++++++++++++++++++++++++
2 files changed, 44 insertions(+)

diff --git a/gdb/riscv-linux-nat.c b/gdb/riscv-linux-nat.c
index d51f6e30218..f0705bc763f 100644
--- a/gdb/riscv-linux-nat.c
+++ b/gdb/riscv-linux-nat.c
@@ -21,6 +21,8 @@
#include "gregset.h"
#include "linux-nat.h"
#include "riscv-tdep.h"
+#include "inferior.h"
+#include "target-descriptions.h"

#include "elf/common.h"

@@ -34,6 +36,9 @@ public:
/* Add our register access methods. */
void fetch_registers (struct regcache *regcache, int regnum) override;
void store_registers (struct regcache *regcache, int regnum) override;
+
+ /* Read suitable target description. */
+ const struct target_desc *read_description () override;
};

static riscv_linux_nat_target the_riscv_linux_nat_target;
@@ -155,6 +160,39 @@ fill_fpregset (const struct regcache *regcache, prfpregset_t *fpregs,
regcache->raw_collect (RISCV_CSR_FCSR_REGNUM, &fpregs->__d.__fcsr);
}

+/* Return a target description for the current target. */
+
+const struct target_desc *
+riscv_linux_nat_target::read_description ()
+{
+ struct riscv_gdbarch_features features;
+ struct iovec iov;
+ elf_fpregset_t regs;
+ int tid;
+
+ /* Figuring out xlen is easy. */
+ features.xlen = sizeof (elf_greg_t);
+
+ tid = inferior_ptid.lwp ();
+
+ iov.iov_base = &regs;
+ iov.iov_len = sizeof (regs);
+
+ /* Can we fetch the f-registers? */
+ if (ptrace (PTRACE_GETREGSET, tid, NT_FPREGSET,
+ (PTRACE_TYPE_ARG3) &iov) == -1)
+ features.flen = 0; /* No f-registers. */
+ else
+ {
+ /* TODO: We need a way to figure out the actual length of the
+ f-registers. We could have 64-bit x-registers, with 32-bit
+ f-registers. For now, just assumed xlen and flen match. */
+ features.flen = features.xlen;
+ }
+
+ return riscv_create_target_description (features);
+}
+
/* Fetch REGNUM (or all registers if REGNUM == -1) from the target
into REGCACHE using PTRACE_GETREGSET. */

[Jim Wilson]

On Wed, Nov 28, 2018 at 2:49 PM Andrew Burgess

Post by Andrew Burgess
Currently this will supply a suitably sized set of x-registers, and
will probe the kernel to see if the f-registers are readable. If they
are readable then we currently assume that the f-registers are the
same size as the x-registers as I don't know of a good way to probe
the f-register length. This will obviously need fixing in future.

The linux kernel currently only supports 64-bit f-registers or none,
so just assuming that they are 64-bits if they exist will work for
now. The NT_FPREGSET ptrace call is only supported if f-registers
exist, so checking for that is fine. My linux kernel patch to add
f-register ptrace support adds an elf_fpreg_t type, but it is in
asm/elf.h which is currently not included, and you can't use the type
if you have a 4.19 or earlier kernel. The elf_gpreg_t definition is
coming from sys/procfs.h which duplicates the definition in asm/elf.h,
but I didn't add an elf_fpreg_t definition to sys/procfs.h, so I'm not
sure what to do about that.

We always have the option of using hwcap info, there are bits set in
there for available architecture extensions. The hwcap support is
apparently broken in some versions of the linux kernel development
tree, but is hopefully correct in all supported releases. There are
macros you can find in asm/hwcap.h. We would have to figure out how
to get these values into gdb, since including asm/hwcap.h directly is
probably not wise.

Jim

[Jim Wilson]

On Wed, Nov 28, 2018 at 2:49 PM Andrew Burgess

Post by Andrew Burgess
Adds riscv_linux_nat_target::read_description method to find a
suitable target description for the native linux target we are running
on.

I tested this on my HiFive Unleashed running a 4.15 kernel with
patches for gcc, gdb, and glibc support. It looks good. It correctly
detects the FP register support. I get 3 extra failures
FAIL: gdb.base/solib-display.exp: NO: continue
FAIL: gdb.base/solib-display.exp: IN: continue
FAIL: gdb.base/solib-display.exp: SEP: continue
which looks a little odd since there is no obvious connection to the
target description support, but this is repeatable so something is
going on here. Anyways, I'm OK with this for now, we can worry about
debugging this problem later.

Jim

[Jim Wilson]

I tried the new patch set on my HiFive Unleashed and it does solve the
problem. The gdb testsuite results on my board look good now.

Jim