Re: [PATCH v2 2/9] RISC-V: Detect XVentanaCondOps from ISA string

Charlie Jenkins
2023-09-25T17:48:18Z

On Mon, Sep 25, 2023 at 07:08:52PM +0530, Anup Patel wrote:
> The Veyron-V1 CPU supports custom conditional arithmetic and
> conditional-select/move operations referred to as XVentanaCondOps
> extension. In fact, QEMU RISC-V also has support for emulating
> XVentanaCondOps extension.
>
> Let us detect XVentanaCondOps extension from ISA string available
> through DT or ACPI.
>
> Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> ---
> arch/riscv/include/asm/hwcap.h | 1 +
> arch/riscv/kernel/cpufeature.c | 1 +
> 2 files changed, 2 insertions(+)
>
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index 0f520f7d058a..b7efe9e2fa89 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -59,6 +59,7 @@
> #define RISCV_ISA_EXT_ZIFENCEI 41
> #define RISCV_ISA_EXT_ZIHPM 42
> #define RISCV_ISA_EXT_SMSTATEEN 43
> +#define RISCV_ISA_EXT_XVENTANACONDOPS 44
>
> #define RISCV_ISA_EXT_MAX 64
>
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 3755a8c2a9de..3a31d34fe709 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -182,6 +182,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
> __RISCV_ISA_EXT_DATA(svinval, RISCV_ISA_EXT_SVINVAL),
> __RISCV_ISA_EXT_DATA(svnapot, RISCV_ISA_EXT_SVNAPOT),
> __RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
> + __RISCV_ISA_EXT_DATA(xventanacondops, RISCV_ISA_EXT_XVENTANACONDOPS),
> };
>
> const size_t riscv_isa_ext_count = ARRAY_SIZE(riscv_isa_ext);
> --
> 2.34.1
>
>
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I worry about storing vendor extensions in this file. Because vendor
extensions are not standardized, they can only be expected to have the
desired behavior on hardware with the appropriate vendor id. A couple
months ago I sent a patch to address this by handling vector extensions
independently for each vendor [1]. I dropped the patch because it
relied upon Heiko's T-Head vector extension support that he stopped
working on. However, I can revive this patch so you can build off of it.

This scheme has the added benefit that vendors do not have to worry
about conficting extensions, and the kernel does not have to act as a
key registry for vendors.

What are your thoughts?

- Charlie

[1] https://lore.kernel.org/lkml/20230705-thead_vendor_extensions-v1-2-ad6915349c4d@rivosinc.com/