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

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Message-ID: <ZgXGKxp0KAQI/+NC@debug.ba.rivosinc.com>
Date: Thu, 28 Mar 2024 12:34:03 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Samuel Holland <samuel.holland@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, tech-j-ext@...ts.risc-v.org,
	Conor Dooley <conor@...nel.org>, kasan-dev@...glegroups.com,
	Evgenii Stepanov <eugenis@...gle.com>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Andrew Jones <ajones@...tanamicro.com>, Guo Ren <guoren@...nel.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [RISC-V] [tech-j-ext] [RFC PATCH 5/9] riscv: Split per-CPU and
 per-thread envcfg bits

On Wed, Mar 27, 2024 at 06:58:45PM -0700, Deepak Gupta via lists.riscv.org wrote:
>On Tue, Mar 19, 2024 at 7:21 PM Samuel Holland
><samuel.holland@...ive.com> wrote:
>>
>> >         else
>> >                 regs->status |= SR_UXL_64;
>> >  #endif
>> > +       current->thread_info.envcfg = ENVCFG_BASE;
>> >  }
>> >
>> > And instead of context switching in `_switch_to`,
>> > In `entry.S` pick up `envcfg` from `thread_info` and write it into CSR.
>>
>> The immediate reason is that writing envcfg in ret_from_exception() adds cycles
>> to every IRQ and system call exit, even though most of them will not change the
>> envcfg value. This is especially the case when returning from an IRQ/exception
>> back to S-mode, since envcfg has zero effect there.
>>
>
>A quick observation:
>So I tried this on my setup. When I put `senvcfg` writes in
>`__switch_to ` path, qemu suddenly
>just tanks and takes a lot of time to boot up as opposed to when
>`senvcfg` was in trap return path.
>In my case entire userspace (all processes) have cfi enabled for them
>via `senvcfg` and it gets
>context switched. Not sure it's specific to my setup. I don't think it
>should be an issue on actual
>hardware.
>
>Still debugging why it slows down my qemu drastically when same writes
>to same CSR
>are moved from `ret_from_exception` to `switch_to`

Nevermind and sorry for the bother. An issue on my setup.

>
>
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