Re: [Qemu-devel] [sw-dev] The problem of write misa on QEMU and BBL

[Andrew Waterman]

On Thu, Apr 19, 2018 at 5:05 PM, Michael Clark <address@hidden> wrote:

>
>
> On Thu, Apr 19, 2018 at 9:28 PM, Zong Li <address@hidden> wrote:
>
>> 2018-04-19 12:43 GMT+08:00 Michael Clark <address@hidden>:
>> > Hi Zong,
>> >
>> >> On 19/04/2018, at 2:40 PM, Zong Li <address@hidden> wrote:
>> >>
>> >> Hi all,
>> >>
>> >> For BBL part, in fp_init at machine/minit.c,
>> >> it will clear the D and F bits of misa register, and assertion that
>> >> the bits is cleared.
>> >> But the misa is WARL register, so there is no effect for writing it,
>> >> and the assertion not be true.
>> >> So is there has necessary to do that if toolchain not support D and F
>> extension?
>> >>
>> >> For QEMU part, when writing misa, it will trigger the illegal
>> >> instruction exception, but I think that the WARL allow write behavior?
>> >
>> > QEMU in the riscv-all branch should have WARL behavior.
>> >
>> > - https://github.com/riscv/riscv-qemu/commits/riscv-all
>> >
>> > There is a bug in upstream. We have submitted patches to fix the issue
>> for review on the qemu-devel mailing list. The patch series will be
>> submitted for upstream review again shortly. We were holding off on the
>> series as we didn’t classify it as a “critical bug” as QEMU was in
>> soft-freeze for 2.12 and we weren’t able to get review in time to include
>> this fix in the 2.12 release.
>> >
>> > See “No traps on writes to misa,minstret,mcycle"
>> >
>> > - https://github.com/riscv/riscv-qemu/commits/qemu-2.13-for-upstream
>> >
>> > The history is that there were several unimplemented CSRs that had
>> printf followed by exit. Richard Henderson said we should fix this. I
>> changed several CSRs to cause illegal instruction traps instead of calling
>> exit. That was a mistake as CSRs that don’t support write are WARL (Write
>> Any Read Legal). It was certainly better than having the simulation exit as
>> a cpu doesn’t typically have a way to ”exit” like a C program, nevertheless
>> trapping was wrong. My mistake. See here for the history:
>> >
>> > - https://github.com/riscv/riscv-qemu/blob/ff36f2f77ec3e6a6211
>> c63bfe1707ec057b12f7d/target-riscv/op_helper.c
>> >
>> > The implementation in the current tree is quite different. We have
>> recently made the CSR system more modular so that with minor changes,
>> custom CPUs will be able to hook their own control and status registers.
>> >
>> > - https://github.com/riscv/riscv-qemu/blob/qemu-2.13-for-upstr
>> eam/target/riscv/csr.c#L780-L867
>> >
>> > See these changes:
>> >
>> > - https://github.com/riscv/riscv-qemu/commit/9d9c1bfef911c520a
>> 35bd3f8c0ed2e14cc96bbb7
>> > - https://github.com/riscv/riscv-qemu/commit/b5a4cd79ce6c7fbb6
>> 5fdcf078fb9a8391da1d6b1
>> >
>> > We know have a flexible system that will allow implementations to hook
>> per-cpu control and status registers, and we have predicates that make CSRs
>> appear on some processor but not on others. i.e. if misa.S is not present,
>> then S-mode s* CSRs will trap. Sometimes WARL is the correct behaviour, but
>> sometimes trapping is the correct behaviour i.e. if the processor does not
>> implement S-mode.
>> >
>> > misa traps on write should only affect bootloaders as Supervisor’s like
>> Linux don’t yet have access to the isa register. It’s not a major issuse.
>> >
>> > Michael.
>>
>> Hi Michael,
>>
>> Thanks for the information. The new CSR system is helpful for custom
>> CPU such as ours. Thanks.
>>
>> In the future, maybe we can do something like this in BBL for flexible
>> custom platform which has own device to control the timer, ipi and so
>> on.
>>
>> Back to the misa problem in BBL, at fp_init in BBL initial phrase, the
>> assertion will has problem because the bits of misa will not be
>> cleared.
>>
>> the code piece like below:
>>     uintptr_t fd_mask = (1 << ('F' - 'A')) | (1 << ('D' - 'A'));
>>     clear_csr(misa, fd_mask);
>>     assert(!(read_csr(misa) & fd_mask));
>>
>> I think that the assertion is not necessary even the clear misa.
>>
>
> I agree. The specification makes no guarantee that misa writes are not
> ignored so it is legal for a processor that supports FD to drop misa writes
> and the assertion will trigger on legal RISC-V implementations. That code
> piece does not support legal RISC-V implementations that can't disable F
> and D. Disabling F and D should not be asserted because it is harmless if
> an unused extension is present.
>

The problem is that BBL cannot cope with this inconsistent scenario.  If pk
is compiled with to assume no floating-point, there had better be no
floating-point.  If you remove the assertion, it will break in other ways
later during in execution.

If you don't want the assertion to fire, compile BBL to match the ISA.


> This assertion will always trigger in QEMU until we support the 'optional'
> feature to allow changes to 'misa'.
>
> Just noting this is not QEMU specifc so we should drop qemu-devel if we
> continue to discuss misa on RISC-V in bbl.
>
> Nevertheless, we do plan to support 'misa' writes however we need to do
> some work in translate.c to make sure that cached translations match the
> current state of misa. We may want to perform a tb_flush when we implement
> writable misa. We also want writable misa to be a CPU feature so we can
> emulate CPUs that don't support writable misa. eg add this to the CPU model.
>
>     set_feature(env, RISCV_FEATURE_MISA_WRITABLE)
>
> Thanks for raising this because the new modular CSR implementation only
> implemented 'existential' predicates for CSRs. We should add a write flag
> to the predicate. Or we can just return -1 in the write_misa function. e.g.
>
>     static int write_misa(CPURISCVState *env, int csrno, target_ulong val)
>     {
>         if (!riscv_feature(env, RISCV_FEATURE_MISA_WRITABLE)) {
>             return -1;
>         }
>         /* validate misa - must contain 'I' or 'E' */
>         env->misa = val;
>         tb_flush(CPU(riscv_env_get_cpu(env)));
>     }
>
> tb_flush is pessimistic but conservative. Currently its not common to
> write misa so it would be acceptable.
>
> There is a similar but somewhat more complex issue for disabling misa.C.
> The behaviour has been discussed on the isa-dev mailing list. Iirc, we have
> to ignore bit 1 in mepc/sepc in MRET/SRET if misa.C has been cleared and a
> 2-byte aligned address is present in mepc/sepc, so that MRET/SRET can only
> jump to 4-byte aligned code. So we drop bit 1 on writes to mepc/sepc while
> misa.C is clear and we ignore bit 1 on reads from mepc/sepc while misa.C is
> cleared. So the change needs slightly more work than just making 'misa'
> writable. We also have to enforce that 'I' or 'E' are set, and we currently
> don't have support for RVE emulation in RISC-V QEMU. This will require
> changes to validate registers in translate.c and cause illegal instructions
> if regno >= 16 is used.
>
> I'm also not sure exactly how we add misa to the translation cache index,
> but tb_flush seems like the conservative way to ensure the translation
> cache matches the currently set bits in misa.
>
> We also have to audit translate.c to make sure that misa is checked for
> all allowable extensions. MAFDC. Currently it only checks 'C' so we will
> need to add checks for 'M' in mul/mulw/div/divw/divu/divuw/rem/remw/remu/remuw
> and 'A' for amos, 'F' and 'D' in floating point operations, etc. It's a
> fair amount of work...
>
> $ grep -r has_ext target/riscv/
> target/riscv//csr.c:    return -!riscv_has_ext(env, RVS);
> target/riscv//csr.c:        (!riscv_has_ext(env, RVS) && mpp == PRV_S) ||
> target/riscv//csr.c:        (!riscv_has_ext(env, RVU) && mpp == PRV_U)) {
> target/riscv//cpu.h:static inline int riscv_has_ext(CPURISCVState *env,
> target_ulong ext)
> target/riscv//op_helper.c:    if (!riscv_has_ext(env, RVC) && (retpc &
> 0x3)) {
> target/riscv//op_helper.c:    if (!riscv_has_ext(env, RVC) && (retpc &
> 0x3)) {
> target/riscv//translate.c:    if (!riscv_has_ext(env, RVC)) {
> target/riscv//translate.c:        if (!riscv_has_ext(env, RVC)) {
> target/riscv//translate.c:    if (!riscv_has_ext(env, RVC) && ((ctx->pc +
> bimm) & 0x3)) {
> target/riscv//translate.c:            if (riscv_has_ext(env, RVS)) {
> target/riscv//translate.c:        if (!riscv_has_ext(env, RVC)) {
>
> So it seems like writable misa is a fair amount of work
>
> - RISCV_FEATURE_MISA_WRITABLE (easy)
> - ISA extension validation rules in write_misa (easy)
> - Extension checks in translate.c (time-consuming but easy)
> - RVC instruction pointer alignment checking rules (needs some care)
> - Make sure we have CPU models with and without writable 'misa' so we can
> test code to handle typical legal processor variants.
>
> Michael
>