linux-kernel - Re: [PATCH] irqchip: riscv-imsic: Add kernel parameter to disable IPIs

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Message-ID: <CAK9=C2Ue36iF8X17n-D70BkMk69bGKb+8dSmro6SEo5i2d+OSg@mail.gmail.com>
Date: Tue, 1 Jul 2025 12:00:09 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Jonathan Corbet <corbet@....net>, Anup Patel <anup@...infault.org>, 
	Atish Patra <atish.patra@...ux.dev>, Palmer Dabbelt <palmer@...belt.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Alexandre Ghiti <alex@...ti.fr>, 
	Andrew Jones <ajones@...tanamicro.com>, linux-doc@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip: riscv-imsic: Add kernel parameter to disable IPIs

On Mon, Jun 30, 2025 at 9:39 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, Jun 25 2025 at 21:47, Anup Patel wrote:
>
> $Subject...
>
> https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject
>
> Is it that hard?

Not sure why I chose a different subject prefix this time. I will update
this in the next revision.

>
> > When injecting IPIs to a set of harts, the IMSIC IPI support will
> > do a separate MMIO write to SETIPNUM_LE register of each target
> > hart. This means on a platform where IMSIC is trap-n-emulated,
> > there will be N MMIO traps when injecting IPI to N target harts
> > hence IPIs based on IMSIC software injected MSI is slow compared
> > to the SBI IPI extension.
> >
> > Add a kernel parameter to disable IPIs in IMSIC driver for platforms
> > with trap-n-emulated IMSIC.
>
> Why do you need a kernel parameter for that. If the platform uses trap-n
> emulation, then disable the IPI muck automatically, no?
>

Unfortunately, we don't have DT, ACPI, or any other way of discovering
whether underlying IMSIC is trap-n-emulated. In fact, the DT or ACPI
passed to a KVM Guest is the same irrespective of whether underlying
IMSIC is trap-n-emulated or backed by hardware IMSIC VS-file.

Using software injected MSIs as IPIs is purely a software choice in the
IMSIC driver so this new kernel parameter allows users to override it.

Regards,
Anup