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

[Daniel Henrique Barboza]

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.

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.

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.


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