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: <CAKC1njTOgsOCpLMCch_YBE+qDoewADhzWwjjfX9nhWCS9hs0mQ@mail.gmail.com>
Date: Wed, 27 Mar 2024 18:58:45 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: 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 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`