#4155 closed defect (fixed)

Add a workaround for the LEON3FT RETT Restart errata: TN-0018

Reported by: Sebastian Huber Owned by: Daniel Hellstrom
Priority: normal Milestone: 6.1
Component: arch/sparc Version: 5
Severity: normal Keywords:
Cc: Blocked By:
Blocking:

Change History (4)

comment:1 Changed on Oct 16, 2020 at 2:58:41 PM by Daniel Hellstrom

This is the proposed GCC10 patch for master, it shuold be applied for RSB/sparc too.

https://lists.rtems.org/pipermail/devel/2020-October/062837.html

comment:2 Changed on Oct 16, 2020 at 3:12:09 PM by Sebastian Huber

The patch looks fine.

comment:4 Changed on Mar 11, 2021 at 4:52:57 PM by Daniel Hellstrom <daniel@…>

Resolution: fixed
Status: assignedclosed

In b2da982/rtems:

leon,tn-0018: work around GRLIB-TN-0018 errata

Overview
========

The errata is worked around in the kernel without requiring toolchain
modifications. It is triggered the JMPL/RETT return from trap instruction
sequence never generated by the compiler and. There are also other
conditions that must must be true to trigger the errata, for example the
instruction that the trap returns to has to be a JMPL instruction. The
errata can only be triggered if certain data is corrected by ECC
(inflicted by radiation), thus it can not be triggered under normal
operation. For more information see:

www.gaisler.com/notes

Affected RTEMS target BSPs:

  • GR712RC
  • UT699
  • UT700/699E

The work around is enabled by defining FIX_LEON3_TN0018 at build time.
After applying the following GCC patch, GCC will set the define when
compiling for an affected multilib:

  • GR712RC (-mcpu=leon3 -mfix-gr712rc)
  • UT700/UT699E (-mpcu=leon3 -mfix-ut700)
  • UT699 (-mcpu=leon -mfix-ut699)

When building for another multilib and TN0018 is still required, it
is possible to enable it on the RTEMS kernel configure line using the
TARGET_CFLAGS (-DFIX_LEON3FT_TN0018) or other by other means.

The following GCC patch sets FIX_LEON3FT_TN0018 for the affected RTEMS
multilibs:


diff --git a/gcc/config/sparc/rtemself.h b/gcc/config/sparc/rtemself.h
index 6570590..ddec98c 100644
--- a/gcc/config/sparc/rtemself.h
+++ b/gcc/config/sparc/rtemself.h
@@ -33,6 +33,8 @@

builtin_assert ("system=rtems"); \
if (sparc_fix_b2bst) \

builtin_define ("FIX_LEON3FT_B2BST"); \

+ if (sparc_fix_gr712rc sparc_fix_ut700 sparc_fix_ut699) \
+ builtin_define ("FIX_LEON3FT_TN0018"); \

} \

while (0)


Workaround Implementation
=========================

In general there are two approaches that the workaround uses:

A) avoid ECC restarting the RETT instruction
B) avoid returning from trap to a JMPL instruction

Where A) comes at a higher performance cost than B), so B) is used
where posssible. B) can be achived for certain returns from trap
handlers if trap entry is controlled by assembly, such as system calls.

A)
A special JMPL/RETT sequence where instruction cache is disabled
temporarily to avoid RETT containing ECC errors, and reading of RETT
source registers to "clean" them from incorrect ECC just before RETT
is executed.

B)
The work around prevents JMPL after system calls (TA instruction) and
modifies assembly code on return from traps jumping back to application
code. Note that for some traps the trapped instruction is always
re-executed and can therefore not trigger the errata, for example the
SAVE instruction causing window overflow or an float instruction causing
FPU disabled trap.

RTEMS SPARC traps workaround implementation:

NAME NOTE TRAP COMMENT

  • window overflow 1 - 0x05 always returns to a SAVE
  • window underflow 1 - 0x06 always returns to a RESTORE
  • interrupt traps 2 - 0x10..1f special rett sequence workaround
  • syscall 3 - 0x80 shutdown system - never returns
  • ABI flush windows 2 - 0x83 special rett sequence workaround
  • syscall_irqdis 4 - 0x89
  • syscall_irqen 4 - 0x8A
  • syscall_irqdis_fp 1 - 0x8B always jumps back to FP instruction
  • syscall_lazy_fp_switch 5 - 0x04 A) jumps back to FP instruction, or to

B) _Internal_error() starting with SAVE

Notes:
1) no workaround needed because trap always returns to non-JMPL instruction
2) workaround implemented by special rett sequence
3) no workaround needed because system call never returns
4) workaround implemented by inserting NOP in system call generation. Thus

fall into 1) when workaround is enabled and no trap handler fix needed.

5) trap handler branches into both 1) and returning to _Internal_error()

which starts with a SAVE and besides since it shuts down the system that
RETT should never be in cache (only executed once) so fix not necessary
in this case.

Any custom trap handlers may also have to be updated. To simplify that,
helper work around assembly code in macros are available in a separate
include file <libcpu/grlib-tn-0018.h>.

Close #4155.

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