linux-kernel - Re: [PATCH 1/1] target/riscv: enable floating point unit

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Message-ID: <620daf0d-caf8-4fe5-901e-2abadb751392@canonical.com>
Date: Wed, 18 Sep 2024 17:49:26 +0200
From: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: Peter Maydell <peter.maydell@...aro.org>,
 Palmer Dabbelt <palmer@...belt.com>,
 Alistair Francis <alistair.francis@....com>, Bin Meng <bmeng.cn@...il.com>,
 Weiwei Li <liwei1518@...il.com>,
 Daniel Henrique Barboza <dbarboza@...tanamicro.com>,
 Liu Zhiwei <zhiwei_liu@...ux.alibaba.com>, qemu-riscv@...gnu.org,
 qemu-devel@...gnu.org, Anup Patel <anup@...infault.org>,
 Atish Patra <atishp@...shpatra.org>, Paul Walmsley
 <paul.walmsley@...ive.com>, Albert Ou <aou@...s.berkeley.edu>,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] target/riscv: enable floating point unit

On 18.09.24 15:56, Andrew Jones wrote:
> Thanks Heinrich, I had also forgotten that distinction. So the last
> question is whether or not we want to reset mstatus.FS to 1 instead of 3,
> as is done in this patch.
> 
> Thanks,
> drew

If the FD flag set to 3 (Dirty) indicates that "the corresponding state 
has potentially been modified since the last context save". Upon context 
switches the value of the floating point registers has to be saved.

The value 1 (Initial) implies that saving the registers on a context 
change is not needed.

3 (Dirty) is the safest choice. This is why OpenSBI is also using it.

For reference:

3.1.6.6. Extension Context Status in mstatus Register
The RISC-V Instruction Set Manual: Volume II - Privileged Architecture
Version 2024-04-11

Best regards

Heinrich