linux-kernel - Re: [PATCH v6 03/29] x86/apic/msi: Set the delivery mode individually for each IRQ

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Message-ID: <8735hmh1f5.ffs@tglx>
Date:   Fri, 06 May 2022 22:05:34 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        x86@...nel.org
Cc:     Tony Luck <tony.luck@...el.com>, Andi Kleen <ak@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Subject: Re: [PATCH v6 03/29] x86/apic/msi: Set the delivery mode
 individually for each IRQ

On Thu, May 05 2022 at 16:59, Ricardo Neri wrote:
> There are no restrictions in hardware to set  MSI messages with its
> own delivery mode.

"messages with its own" ? Plural/singular confusion.

> Use the mode specified in the provided IRQ hardware
> configuration data. Since most of the IRQs are configured to use the
> delivery mode of the APIC driver in use (set in all of them to
> APIC_DELIVERY_MODE_FIXED), the only functional changes are where
> IRQs are configured to use a specific delivery mode.

This does not parse. There are no functional changes due to this patch
and there is no point talking about functional changes in subsequent
patches here.

> Changing the utility function __irq_msi_compose_msg() takes care of
> implementing the change in the in the local APIC, PCI-MSI, and DMAR-MSI

in the in the

> irq_chips.
>
> The IO-APIC irq_chip configures the entries in the interrupt redirection
> table using the delivery mode specified in the corresponding MSI message.
> Since the MSI message is composed by a higher irq_chip in the hierarchy,
> it does not need to be updated.

The point is that updating __irq_msi_compose_msg() covers _all_ MSI
consumers including IO-APIC.

I had to read that changelog 3 times to make sense of it. Something like
this perhaps:

  "x86/apic/msi: Use the delivery mode from irq_cfg for message composition

   irq_cfg provides a delivery mode for each interrupt. Use it instead
   of the hardcoded APIC_DELIVERY_MODE_FIXED. This allows to compose
   messages for NMI delivery mode which is required to implement a HPET
   based NMI watchdog.

   No functional change as the default delivery mode is set to
   APIC_DELIVERY_MODE_FIXED."

Thanks,

        tglx