linux-kernel - Re: [PATCH 2/2] powerpc/pseries: pass MSI affinity to irq_create

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Date:   Tue, 24 Nov 2020 23:35:34 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Laurent Vivier <lvivier@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-block@...r.kernel.org,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Marc Zyngier <maz@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-pci@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Laurent Vivier <lvivier@...hat.com>
Subject: Re: [PATCH 2/2] powerpc/pseries: pass MSI affinity to irq_create_mapping()

On Tue, Nov 24 2020 at 21:03, Laurent Vivier wrote:
> With virtio multiqueue, normally each queue IRQ is mapped to a CPU.
>
> This problem cannot be shown on x86_64 for two reasons:

There is only _ONE_ reason why this is not a problem on x86. x86 uses
the generic PCI/MSI domain which supports this out of the box.

> - the call path traverses arch_setup_msi_irqs() that is arch specific:
>
>    virtscsi_probe()
>       virtscsi_init()
>          vp_modern_find_vqs()
>             vp_find_vqs()
>                vp_find_vqs_msix()
>                   pci_alloc_irq_vectors_affinity()
>                      __pci_enable_msix_range()
>                         pci_msi_setup_msi_irqs()
>                            arch_setup_msi_irqs()
>                               rtas_setup_msi_irqs()

This is a problem on _all_ variants of PPC MSI providers, not only for
pseries. It's not restricted to virtscsi devices either, that's just the
device which made you discover this.

Thanks,

        tglx