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)

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Deepak Gupta <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Acked-by: Palmer Dabbelt <[email protected]>
Tested-by: Hsieh-Tseng Shen <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Glenn Washburn <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Jeff Xie <[email protected]>
Cc: Kieran Bingham <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions