linux-kernel - Re: [patch 21/32] NTB/msi: Convert to msi_on_each

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Message-ID: <871r2w9y3x.ffs@tglx>
Date:   Wed, 01 Dec 2021 15:52:02 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Marc Zygnier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Megha Dey <megha.dey@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-pci@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>, x86@...nel.org
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

On Tue, Nov 30 2021 at 22:23, Thomas Gleixner wrote:
> On Tue, Nov 30 2021 at 16:28, Jason Gunthorpe wrote:
>
> The real problem is where to store the MSI descriptors because the PCI
> device has its own real PCI/MSI-X interrupts which means it still shares
> the storage space.

Bah. I confused myself by staring at the existing code instead of
looking at how this NTB stuff actually works.

So if I understand it correctly then the end result looks like this:

1) PCIe device (switchtec)

   The device has 4 MSI[X] interrupts: event, dma_rpc, message,
   doorbell. The event and dma_rpc interrupts are requested by the
   switchtec PCI driver itself.

2) Switchtec character device

   The switchtec PCI driver creates a character device which is exposed
   for device specific IOCTLs

   The device belongs to the switchtec_class device class.

3) Switchtec NTB device

   The ntb_hw_switchtec driver registers the switchtec_class class
   interface.

   So when #2 is registered with the driver core the switchtec class
   interface add_dev() function is invoked. That function creates a NTB
   device, requests the message and the doorbell interrupts which have
   been allocated by the underlying PCIe device driver (#1) and
   registers the NTB device with the NTB core.

4) The NTB core then tries to use the virtual MSI vectors which have
   been allocated by the switchtec driver in #1 and requires the msg
   write intercept to actually expose it to the peers.

So we really can go and create a MSI irqdomain and stick the pointer
into stdev->dev.irqdomain. The parent domain of this irqdomain is

     stdev->pdev.dev.irqdomain->parent

which is either the irq remapping domain or the vector domain. Which is
pretty much what I proposed as general facility for IMS/IDXD. I need to
go back and polish that up on top of the current pile.

Along with that have an irq chip implementation which exposes:

static struct irq_chip ntb_chip = {
	.name			= "ntb",
	.irq_ack		= irq_chip_ack_parent,
	.irq_write_msi_msg	= ntb_msi_write_msg,
#ifdef CONFIG_SMP
	.irq_set_affinity	= irq_chip_set_affinity_parent,
#endif
};

We just need some reasonable solution for the DMA/remap problem Jason
mentioned vs. msi_desc::dev, but that wants to be cleaned up in any
case for all the aliasing muck.

Thanks,

        tglx