- If an interrupt arrives at the same time as a load/store instruction
is in ID stage, the interrupt must wait until load/store completes.
Without the WB stage this happens naturally as the core stalls. With
the WB stage, we need to allow the load/store to progress to the WB
stage (and clear the ID stage) then hold back the interrupt until it
completes.
- Also cleaned up some lsu related stalling terms and signal naming.
Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
When a potential exception occurs in ID/EX controller must wait for any
outstanding instruction in WB to complete before resolving it. The
instruction in WB may also have an exception which takes priority over
ID/EX.
Signed-off-by: Greg Chadwick <gac@lowrisc.org>
With the writeback stage enabled the controller can see a load or store
error from the writeback stage whilst seeing some other fault/exception
from ID/EX (e.g. an instruction fetch error). The writeback stage fault
must take priority, however without the writeback stage the
priortisation changes.
This introduces more explicit prioritisation logic for faults/exceptions
and gives the correct prioritisation for configurations both with and
without a writeback stage.
Fixes#912
Signed-off-by: Greg Chadwick <gac@lowrisc.org>
- Drive a speculative version of the branch signal into the IF stage to
drive address muxing
- The speculative signal is the same as the regular branch signal but
assumes all conditional branches are taken
- This breaks the timing path from branch condition calculation into
address muxing (and therefore PMP error calculation)
- When the branch is not taken, any external request we might otherwise
have made is suppressed
- This has a minor performance cost (0.8% without I$, ~0% with I$)
Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
- The controller state machine could only progress to FLUSH to handle an
exception if instr_valid_i was set
- When the exception comes from a load/store in the Writeback stage, and
no new instruction has been driven into the ID stage, this could cause
exception to be missed
- The instr_valid_i qualification is therefore removed from the state
machine as all relevant signals inside that if block are already
qualified by instr_valid_i anyway
- Fixes#849
Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
- Before this fix, the branch signal was qualified by the illegal
instruction signal and the illegal csr signal.
- This patch removes both of these since the decoder already masks
branches with illegal isntruction, and a branch cannot be a CSR op.
- This improves the worst path in the design significantly without the
branch target ALU.
Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
mtval should record which half of the instruction caused the error
rather than just recording the PC.
An extra signal is added in the IF stage to indicate when an error is
caused by the second half of an unaligned instruction. This signal is
then used to increment the PC by 2 for mtval capture on an error.
Fixes#709
The third pipeline stage is a new writeback stage. Ibex can now be
configured as the original two stage design or the new three stage
design using the `WritebackStage` parameter in ibex_core. This defaults
to 0 (giving the original two stage design).
The three stage design is *EXPERIMENTAL*
In the three stage design all register write back occurs in the third,
final stage. This allows a cycle for responses to loads and stores so
when the memory system can respond in a single cycle there will be no
stall. This offers significant performance benefits.
Documentation of the three stage design is still to be written so
existing documentation applies to the two stage design only as various
aspects of Ibex behaviour will change in the three stage design.
Signed-off-by: Greg Chadwick <gac@lowrisc.org>
This commit modifies the `mip` CSR to not depend on the `mie` CSR. While
the values of both these CSRs are combined to decide whether an
interrupt shall be handled, the RISC-V spec does not state that the
content of of `mip` should depend on `mie`. This commit better aligns
Ibex with other open-source RISC-V cores.
This resolveslowRISC/ibex#567 reported by @pfmooney.
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
prim_assert.sv is a file containing assertion macros (defines).
Previously, prim_assert.sv was compiled as normal SystemVerilog file.
This made the defines available for the whole compilation unit as soon
as they were defined. Since all cores using prim_assert depended (in
fusesoc) on the lowrisc:prim:assert core, prim_assert was always
compiled first, and the defines were visible in subsequent files.
All of that is only true if all files end up in one comilation unit. The
SV standard states that what makes up a compilation unit is
tool-defined, but also states that typically, passing multiple files (or
a file list/.f file) to a single tool invocation means that all files
end up in one compilation unit; if the tool is called multiple times,
then the files end up in separate compilation units.
Edalize (the fusesoc backend) doesn't guarantee either behavior, and so
it happens that for Vivado, Verilator, Cadence and Synopsys simulators,
all files are compiled into a single compilation unit. But for Riviera,
each file is a separate compilation unit.
To avoid relying on the definition of compilation units, and to do the
generally right thing (TM), this commit changes the code to always
include the prim_assert.sv file when it is used in a source file.
Include guards are introduced in the prim_assert.sv file to avoid
defining things twice.
This commit replaces all X assignments in the RTL with defined
values. In addition, SystemVerilog Assertions are added to catch
invalid signal values in simulation. A new file containing the
corresponding assertion macros is added as well.
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
- Add the minimum amount of trigger system to support GDB hbreak
- Only a single trigger is implemented
- Only instruction address matching
- Only break into debug mode (no native debug)
- Fixes#382
Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
This commit reworks the generation of the `core_busy` signal used to
control the main clock gate of the core. Without this commit, the
controller generates a separate `first_fetch` signal only asserted in
the FIRST_FETCH state that directly controls `core_busy` and thus the
main clock gate. This is problematic as it introduces a feedback to
from the controller state into the clock.
This commit removes the problematic signal and changes the generation of
`ctrl_busy` in the FIRST_FETCH state of the controller. This signal is
now used to control the main clock gate in all states (previously all
except FIRST_FETCH) but it gets registered, thus it does not introduce
the feedback into the clock.
This resolveslowRISC/ibex#211.
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
This commit removes a redundant for `ebrk_insn` in the `DBG_TAKEN_ID`
state. If this signal is not set, the controller does not enter this
state in the first place. This is not changing functional behavior,
but removes a `MISSING_ELSE` coverage hole reported by @udinator.
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
This commit turns the last case in the exception handling if/else block
from an `else if` to an `else`. This is not changing functional
behavior as the same condition is checked previously, but removes a
`MISSING_ELSE` coverage hole reported by @udinator.
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
When we single step over an instruction that causes an exception DPC
should be set to point to the exception handler (where we would have
gone were we not single stepping).
Fixes#393
When entering debug mode in controller need to ensure ID doesn't get
flushed until the relevant debug state can see the contents of ID and
perform the appropriate actions. ID is flushed at that point.
Also cleaned up some code replication around entering debug mode
(debug_req vs single step look very similar so can use the same code
paths).
When an exception occurs controller goes into the flush state and then
does a pc_set to go the exception handler. So there is no need for the
incoming signals that indicate an exception to factor directly into
pc_set_o. This flops exc_req and illegal_insn to break long timing
paths that were occurring because of this.
Fixes#305
When an instruction that caused an exception occurred the controller
would clear the instr_valid bit but rely on the instruction bits staying
in the ID register so it could still use them in the FLUSH state to
decide what to do. This reworks the control logic slightly so
instr_valid remains asserted when the controller goes into the FLUSH
state so relevant signals can be qualified with the instr_valid bit.
There were no known functional issues caused by the previous behaviour
however this gives a more robust approach.
Without this commit, the `nmi_mode` register is continuously cleared and
set again, and the core can never handle the NMI.
This resolveslowRISC/ibex#300 reported by @udinator.
This commit fixes the generation of the exception cause ID for fast
interrupts. Without this commit, the core tries to handle fast
interrupts using the handlers of other interrupts like external,
software, and timer interrupts.
This commit resolveslowRISC/ibex#290 reported by @udinator.
- The RISC-V Debug Spec v.0.13.2 (p.41) mandates that the core
debug CSRs dcsr, dpc, dscratch0 and dscratch1 must not be
accessible if not in debug mode. Fixes#275
Executing a DRET instruction outside of debug mode now causes an
illegal instruction exception as mandated by the Debug Spec v0.13.2
p.41.
This resolveslowRISC/ibex#270.
We currently have a documentation block at the beginning of each file,
containing author credits and module-level documentation. The
module-level documentation is retained for historic reasons and
duplicated with the newer comments below it.
For the authors, maintaining author credits in the file is error-prone,
as this information gets outdated very soon. A more reliable way to see
who modified a file is to use the history information in git.
Additionally, we now have the CREDITS.md file, which lists all
contributors, even the ones which don't appear in the git history (e.g.
because the code was copied and commited by someone else).
This commit clarifies why CSR-related pipeline flushes are needed
(e.g. when enabling interrupts), only introduces them when doing the
critical modifications (write/set bits in `mstatus` and `mie` CSRs
for enabling interrupts, reads and clears are uncritical for these
CSRs), and makes sure the controller is actually able to start
handling interrupts while doing a CSR-related pipeline flush.
This resolveslowrisc/ibex#6.
- Add required signals to top-level
- Propagate error through fetch stages
- Add new exception type
- Update documentation for new exception type
- Resolves issue #109
This commit adds 15 local fast interrupt lines managed through
the `mie` and `mip` CSRs directly, and removes the old legacy
interrupts including the internal controller.
This commit adds the `mip` and `mie` CSRs as well as the software,
timer and external interrupts.
Currently these interrupts are overlapped with some of the legacy
interrupts, i.e., the core will jump to the same address for different
interrupts. For example, the software interrupt has the same ID as
legacy interrupt #3. Thus, the handler must read the `mip` register
to find out whether the interrupt routine has been entered because of
the software interrupt or the legacy interrupt.
Eventually, the number of legacy interrupts will be reduced to avoid
this overlapping, and they will be connected to the `mie` and `mip`
CSRs directly.
This file doesn't contain defines any more, but a normal SV package.
The diff is best viewed without whitespace changes, as the reindents
cause a lof of diff noise.
Fixeslowrisc/ibex#173
This commit changes the controller to save the PC of the next
instruction to the dpc CSR when entering debug mode upon a halt
request instead of the PC of the current instruction (only valid
for EBREAK instructions).
This bug was reported by @taoliug. This resolves#164.
The RVFI implementation make use of signals from the decoder and
controller to detect the state of the processor.
Especially the signal for a new and retired instruction.
* Controller: remove impossible condition for `DBG_TAKEN_IF`
There is no way to jump into `DBG_TAKEN_IF` because of an EBREAK
instruction. Thus, this case also does not need to be checked.
* Controller: do not enter debug when `debug_req_i` goes low
With this commit, the core is prevented from entering debug mode when
the debug request signal is deasserted during that procedure.
Previously, the core would still enter debug mode but not updating the
debug CSR.
This resolves#153 reported by @taoliug.
* Update ibex_controller.sv
The FSM now uses input signals for interrupt and debug request handling
that depend on whether the core is in debug mode. This avoids the need
for many `debug_mode_q` guards and somewhat simplifies the FSM.
If in debug mode, the core shall not handle interrupts according to the
spec anyway.
