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

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Date:   Wed, 11 May 2022 17:38:45 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, 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
Subject: Re: [PATCH v6 03/29] x86/apic/msi: Set the delivery mode
 individually for each IRQ

On Fri, May 06, 2022 at 10:05:34PM +0200, Thomas Gleixner wrote:
> 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.

Yes, this is not correct. It should have read "messages with their own..."

> 
> > 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.

I will remove this.

> 
> > 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

Sorry! This is not correct.

> 
> > 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."

Thank you for your help on the changelog! I will take your suggestion.

BR,
Ricardo
> 
> Thanks,
> 
>         tglx