Re: [PATCH RFC 0/8] Add Counter delegation ISA extension support

[Atish Kumar Patra <atishp@rivosinc.com>]

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On Wed, Feb 21, 2024 at 6:58 AM Daniel Henrique Barboza <
dbarboza@ventanamicro.com> wrote:

> Hi Atish,
>
> This series and its dependency, which I assume it's
>
> "[PATCH v4 0/5] Add ISA extension smcntrpmf support"
>
> Doesn't apply in neither master nor riscv-to-apply.next because of this
> patch:
>
"target/riscv: Use RISCVException as return type for all csr ops"
>
> That changed some functions from 'int' to "RISCVException" type. The
> conflicts
> from the v4 series are rather trivial but the conflicts for this RFC are
> annoying
> to deal with. It would be better if you could re-send both series rebased
> with
> the latest changes.
>
>
I was waiting for Alistair's ACK on the smcntrpmf series as he had some
comments. It looks like he is okay
with the series now (no further questions).  Let me respin both the series.


> One more thing:
>
> On 2/16/24 21:01, Atish Patra wrote:
> > This series adds the counter delegation extension support. The counter
> > delegation ISA extension(Smcdeleg/Ssccfg) actually depends on multiple
> ISA
> > extensions.
> >
> > 1. S[m|s]csrind : The indirect CSR extension[1] which defines additional
> >     5 ([M|S|VS]IREG2-[M|S|VS]IREG6) register to address size limitation
> of
> >     RISC-V CSR address space.
> > 2. Smstateen: The stateen bit[60] controls the access to the registers
> >     indirectly via the above indirect registers.
> > 3. Smcdeleg/Ssccfg: The counter delegation extensions[2]
> >
> > The counter delegation extension allows Supervisor mode to program the
> > hpmevent and hpmcounters directly without needing the assistance from the
> > M-mode via SBI calls. This results in a faster perf profiling and very
> > few traps. This extension also introduces a scountinhibit CSR which
> allows
> > to stop/start any counter directly from the S-mode. As the counter
> > delegation extension potentially can have more than 100 CSRs, the
> specificaiton
> > leverages the indirect CSR extension to save the precious CSR address
> range.
> >
> > Due to the dependancy of these extensions, the following extensions must
> be
> > enabled to use the counter delegation feature in S-mode.
> >
> >
> "smstateen=true,sscofpmf=true,ssccfg=true,smcdeleg=true,smcsrind=true,sscsrind=true"
> >
> > This makes the qemu command line quite tedious. In stead of that, I
> think we
> > can enable these features by default if there is no objection.
>
> It wasn't need so far but, if needed, we can add specialized setters for
> extensions
> that has multiple dependencies. Instead of the usual setter we would do
> something
> like:
>
> cpu_set_ssccfg() {
>
>      if (enabled) {
>          smstateen=true
>          sscofpmf=true
>          smcdeleg=true
>          smcsrind=true
>          sscsrind=true
>      }
> }
>
>
> The advantage is that this setter would also work for CPUs that doesn't
> inherit defaults,
> like bare-cps and profile CPUs.
>
>
Your suggested approach looks good to me. But I was asking about concerns
about enabling these extensions
by default rather than the actual mechanism to implement it. Few of the
extensions listed here such as smstateen,smcsrind
sscsrind are independent ISA extensions which are used for other ISA
extensions as well.

It looks like you are okay with the use case also ?


> That doesn't mean we can't add defaults for rv64, but for this particular
> case I wonder if
> the 'max' CPU wouldn't be better.
>
>
Not sure what you mean here. What does 'max' cpu have to do with pmu
extensions ?


>
> Thanks,
>
>
> Daniel
>
> >
> > The first 2 patches decouple the indirect CSR usage from AIA
> implementation
> > while patch3 adds stateen bits validation for AIA.
> > The PATCH4 implements indirect CSR extensions while remaining patches
> > implement the counter delegation extensions.
> >
> > The Qemu patches can be found here:
> > https://github.com/atishp04/qemu/tree/counter_delegation_rfc
> >
> > The opensbi patch can be found here:
> > https://github.com/atishp04/opensbi/tree/counter_delegation_v1
> >
> > The Linux kernel patches can be found here:
> > https://github.com/atishp04/linux/tree/counter_delegation_rfc
> >
> > [1] https://github.com/riscv/riscv-indirect-csr-access
> > [2] https://github.com/riscv/riscv-smcdeleg-ssccfg
> >
> > Atish Patra (1):
> > target/riscv: Enable S*stateen bits for AIA
> >
> > Kaiwen Xue (7):
> > target/riscv: Add properties for Indirect CSR Access extension
> > target/riscv: Decouple AIA processing from xiselect and xireg
> > target/riscv: Support generic CSR indirect access
> > target/riscv: Add smcdeleg/ssccfg properties
> > target/riscv: Add counter delegation definitions
> > target/riscv: Add select value range check for counter delegation
> > target/riscv: Add counter delegation/configuration support
> >
> > target/riscv/cpu.c      |   8 +
> > target/riscv/cpu.h      |   1 +
> > target/riscv/cpu_bits.h |  34 +-
> > target/riscv/cpu_cfg.h  |   4 +
> > target/riscv/csr.c      | 713 +++++++++++++++++++++++++++++++++++++---
> > target/riscv/machine.c  |   1 +
> > 6 files changed, 722 insertions(+), 39 deletions(-)
> >
> > --
> > 2.34.1
> >
>

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