Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits

From: Andrew Jones <ajones@ventanamicro.com>
To: Samuel Holland <samuel.holland@sifive.com>
Cc: Deepak Gupta <debug@rivosinc.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	 Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org, tech-j-ext@lists.risc-v.org,
	 Conor Dooley <conor@kernel.org>,
	kasan-dev@googlegroups.com,
	 Evgenii Stepanov <eugenis@google.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	 Rob Herring <robh+dt@kernel.org>, Guo Ren <guoren@kernel.org>,
	Heiko Stuebner <heiko@sntech.de>,
	 Paul Walmsley <paul.walmsley@sifive.com>
Subject: Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and per-thread envcfg bits
Date: Fri, 22 Mar 2024 08:58:43 +0100	[thread overview]
Message-ID: <20240322-168f191eeb8479b2ea169a5e@orel> (raw)
In-Reply-To: <40ab1ce5-8700-4a63-b182-1e864f6c9225@sifive.com>

On Tue, Mar 19, 2024 at 09:20:59PM -0500, Samuel Holland wrote:
...
> This is really a separate concern than when we write envcfg. The per-CPU
> variable is only necessary to support hardware where a subset of harts support
> Zicboz. Since the riscv_cpu_has_extension_[un]likely() helpers were added
> specifically for Zicboz, I assume this is an important use case, and dropping
> support for this hardware would be a regression. After all, hwprobe() allows
> userspace to see that Zicboz is implemented at a per-CPU level. Maybe Andrew can
> weigh in on that.
>

Hi Samuel,

I've approached Zicboz the same way I would approach all extensions, which
is to be per-hart. I'm not currently aware of a platform that is / will be
composed of harts where some have Zicboz and others don't, but there's
nothing stopping a platform like that from being built. I realize this
adds complexity that we may not want to manage in Linux without an actual
use case requiring it. I wouldn't be opposed to keeping things simple for
now, only bringing in complexity when needed (for this extension or for a
future extension with envcfg bits), but we should ensure we make it clear
that we're making those simplifications now based on assumptions, and we
may need to change things later.

Thanks,
drew