Re: [PATCH v6 1/9] target/riscv: turn write_misa() into an official no-op

[Andrew Jones]

On Tue, Feb 21, 2023 at 12:49:11PM -0300, Daniel Henrique Barboza wrote:
> Hey,
> 
> On 2/16/23 22:42, LIU Zhiwei wrote:
> > 
> > On 2023/2/17 5:55, Daniel Henrique Barboza wrote:
> > > At this moment, and apparently since ever, we have no way of enabling
> > > RISCV_FEATURE_MISA. This means that all the code from write_misa(), all
> > > the nuts and bolts that handles how to write this CSR, has always been a
> > > no-op as well because write_misa() will always exit earlier.
> > > 
> > > This seems to be benign in the majority of cases. Booting an Ubuntu
> > > 'virt' guest and logging all the calls to 'write_misa' shows that no
> > > writes to MISA CSR was attempted. Writing MISA, i.e. enabling/disabling
> > > RISC-V extensions after the machine is powered on, seems to be a niche
> > > use.
> > > 
> > > Before proceeding, let's recap what the spec says about MISA. It is a
> > > CSR that is divided in 3 fields:
> > > 
> > > - MXL, Machine XLEN, described as "may be writable";
> > > 
> > > - MXLEN, the XLEN in M-mode, which is given by the setting of MXL or a
> > > fixed value if MISA is zero;
> > > 
> > > - Extensions is defined as "a WARL field that can contain writable bits
> > > where the implementation allows the supported ISA to be modified"
> > > 
> > > Thus what we have today (write_misa() being a no-op) is already a valid
> > > spec implementation. We're not obliged to have a particular set of MISA
> > > writable bits, and at this moment we have none.
> > 
> > Hi Daniel,
> > 
> > I see there has been a discussion on this topic. And as no-op has no 
> > harmfulness for current implementation.
> > However, I still think we should make misa writable as default, which is 
> > also a valid spec implementation.
> > 
> > One reason is that may be we need to dynamic write  access for some cpus in 
> > the future. The other is we should
> > make QEMU a more useful implementation, not just a legal implementation. We 
> > have done in many aspects on this direction.
> > 
> > I prefer your implementation before v4. It's not a complicated 
> > implementation. And I think the other extensions on QEMU currently
> > can mostly be configurable already.
> 
> I don't have a strong opinion in this matter to be honest. My problems with 
> the
> existing code are:
> 
> - the code is untested. I cannot say that this was never tested, but I can 
> say that
> this has been mostly untested ever since introduced. Which is normal for a 
> code that
> is 'dormant'.
> 
> - the code is dormant and most likely with bugs, but it's still maintained. 
> For
> example we have e91a7227 ("target/riscv: Split misa.mxl and misa.ext") that 
> had
> to make changes here. So we have the upkeep but no benefits.
> 
> - we don't have an use case for it. Most OSes doesn't seem to care, and afaik 
> no
> applications seems to care either.
> 
> 
> All this said, I think we can reach a consensus of keeping it if we can at 
> least come
> up with a way of testing it.
> 
> 
> > 
> > Your work is a good step towards to unify the configuration and the check.  
> > I think two more steps we can go further.
> > 
> > 1) Remove RVI/RVF and the similar macros, and add fields for them in the 
> > configuration struct.
> > 
> > 2) Unify the check about configuration. write_misa and cpu_realize_fn can 
> > use the same check function.
> > 
> > 
> > As we have done these two steps, I think we can go more closely for the 
> > profile extension.
> 
> 
> Is this the extension you're taking about?
> 
> https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc
> 
> 
> This looks like a good reason to keep the code. Let's see if anyone else has 
> an opinion
> about it. We can do the improvements you mentioned above as a follow-up (this 
> series was
> really about removing RISC_FEATURE_*) if we decide to keep it.

If we decide to keep it and not guard it by default, then we should test
and fix it now. Also, as we're already aware that it has insufficient
sanity checks for extension dependencies, then we should fix our general
extension dependency checking now too, in order to apply that to this.
IOW, trying to keep this, without some guard on it, opens a can of worms.
My vote is the same as it was before, merge this series and then revisit
this function when someone has a use/test case for it. Nobody said this
was never going to have a different implementation, just that the current
implementation is known-buggy and there's no reason to expose it now.

My only concern with the code deletion is that git-blame doesn't blame
deleted code. I think we should add a comment describing the history
which includes a git commit reference which can be used to see the
latest implementation.

Thanks,
drew

> 
> 
> 
> Thanks,
> 
> 
> Daniel
> 
> > 
> > 
> > Zhiwei
> > 
> > > Given that allowing the dormant code to write MISA can cause tricky bugs
> > > to solve later on, and we don't have a particularly interesting case of
> > > writing MISA to support today, and we're already not violating the
> > > specification, let's erase all the body of write_misa() and turn it into
> > > an official no-op instead of an accidental one. We'll keep consistent
> > > with what we provide users today but with 50+ less lines to maintain.
> > > 
> > > RISCV_FEATURE_MISA enum is erased in the process since there's no one
> > > else using it.
> > > 
> > > Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> > > Reviewed-by: Bin Meng <bmeng@tinylab.org>
> > > Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> > > ---
> > >   target/riscv/cpu.h |  1 -
> > >   target/riscv/csr.c | 55 ----------------------------------------------
> > >   2 files changed, 56 deletions(-)
> > > 
> > > diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> > > index 7128438d8e..01803a020d 100644
> > > --- a/target/riscv/cpu.h
> > > +++ b/target/riscv/cpu.h
> > > @@ -89,7 +89,6 @@ enum {
> > >       RISCV_FEATURE_MMU,
> > >       RISCV_FEATURE_PMP,
> > >       RISCV_FEATURE_EPMP,
> > > -    RISCV_FEATURE_MISA,
> > >       RISCV_FEATURE_DEBUG
> > >   };
> > > diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> > > index 1b0a0c1693..f7862ff4a4 100644
> > > --- a/target/riscv/csr.c
> > > +++ b/target/riscv/csr.c
> > > @@ -1329,61 +1329,6 @@ static RISCVException read_misa(CPURISCVState 
> > > *env, int csrno,
> > >   static RISCVException write_misa(CPURISCVState *env, int csrno,
> > >                                    target_ulong val)
> > >   {
> > > -    if (!riscv_feature(env, RISCV_FEATURE_MISA)) {
> > > -        /* drop write to misa */
> > > -        return RISCV_EXCP_NONE;
> > > -    }
> > > -
> > > -    /* 'I' or 'E' must be present */
> > > -    if (!(val & (RVI | RVE))) {
> > > -        /* It is not, drop write to misa */
> > > -        return RISCV_EXCP_NONE;
> > > -    }
> > > -
> > > -    /* 'E' excludes all other extensions */
> > > -    if (val & RVE) {
> > > -        /* when we support 'E' we can do "val = RVE;" however
> > > -         * for now we just drop writes if 'E' is present.
> > > -         */
> > > -        return RISCV_EXCP_NONE;
> > > -    }
> > > -
> > > -    /*
> > > -     * misa.MXL writes are not supported by QEMU.
> > > -     * Drop writes to those bits.
> > > -     */
> > > -
> > > -    /* Mask extensions that are not supported by this hart */
> > > -    val &= env->misa_ext_mask;
> > > -
> > > -    /* Mask extensions that are not supported by QEMU */
> > > -    val &= (RVI | RVE | RVM | RVA | RVF | RVD | RVC | RVS | RVU | RVV);
> > > -
> > > -    /* 'D' depends on 'F', so clear 'D' if 'F' is not present */
> > > -    if ((val & RVD) && !(val & RVF)) {
> > > -        val &= ~RVD;
> > > -    }
> > > -
> > > -    /* Suppress 'C' if next instruction is not aligned
> > > -     * TODO: this should check next_pc
> > > -     */
> > > -    if ((val & RVC) && (GETPC() & ~3) != 0) {
> > > -        val &= ~RVC;
> > > -    }
> > > -
> > > -    /* If nothing changed, do nothing. */
> > > -    if (val == env->misa_ext) {
> > > -        return RISCV_EXCP_NONE;
> > > -    }
> > > -
> > > -    if (!(val & RVF)) {
> > > -        env->mstatus &= ~MSTATUS_FS;
> > > -    }
> > > -
> > > -    /* flush translation cache */
> > > -    tb_flush(env_cpu(env));
> > > -    env->misa_ext = val;
> > > -    env->xl = riscv_cpu_mxl(env);
> > >       return RISCV_EXCP_NONE;
> > >   }