On A-profile, PSR bits [15:10][26:25] are always the IT state bits.
On M-profile, some of the reserved encodings of the IT state are used
to instead indicate partial progress through instructions that were
interrupted partway through by an exception and can be resumed.
These resumable instructions fall into two categories:
(1) load/store multiple instructions, where these bits are called
"ICI" and specify the register in the ldm/stm list where execution
should resume. (Specifically: LDM, STM, VLDM, VSTM, VLLDM, VLSTM,
CLRM, VSCCLRM.)
(2) MVE instructions subject to beatwise execution, where these bits
are called "ECI" and specify which beats in this and possibly also
the following MVE insn have been executed.
There are also a few insns (LE, LETP, and BKPT) which do not use the
ICI/ECI bits but must leave them alone.
Otherwise, we should raise an INVSTATE UsageFault for any attempt to
execute an insn with non-zero ICI/ECI bits.
So far we have been able to ignore ECI/ICI, because the architecture
allows the IMPDEF choice of "always restart load/store multiple from
the beginning regardless of ICI state", so the only thing we have
been missing is that we don't raise the INVSTATE fault for bad guest
code. However, MVE requires that we honour ECI bits and do not
rexecute beats of an insn that have already been executed.
Add the support in the decoder for handling ECI/ICI:
* identify the ECI/ICI case in the CONDEXEC TB flags
* when a load/store multiple insn succeeds, it updates the ECI/ICI
state (both in DisasContext and in the CPU state), and sets a flag
to say that the ECI/ICI state was handled
* if we find that the insn we just decoded did not handle the
ECI/ICI state, we delete all the code that we just generated for
it and instead emit the code to raise the INVFAULT. This allows
us to avoid having to update every non-MVE non-LDM/STM insn to
make it check for "is ECI/ICI set?".
We continue with our existing IMPDEF choice of not caring about the
ICI state for the load/store multiples and simply restarting them
from the beginning. Because we don't allow interrupts in the middle
of an insn, the only way we would see this state is if the guest set
ICI manually on return from an exception handler, so it's a corner
case which doesn't merit optimisation.
ICI update for LDM/STM is simple -- it always zeroes the state. ECI
update for MVE beatwise insns will be a little more complex, since
the ECI state may include information for the following insn.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
v1->v2: minor changes as suggested by rth