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

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Date:   Mon, 29 Nov 2021 20:01:50 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

On Mon, Nov 29, 2021 at 04:52:35PM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2021-11-29 4:31 p.m., Jason Gunthorpe wrote:
> > On Mon, Nov 29, 2021 at 03:27:20PM -0700, Logan Gunthorpe wrote:
> > 
> >> In most cases, the NTB code needs more interrupts than the hardware
> >> actually provides for in its MSI-X table. That's what PCI_IRQ_VIRTUAL is
> >> for: it allows the driver to request more interrupts than the hardware
> >> advertises (ie. pci_msix_vec_count()). These extra interrupts are
> >> created, but get flagged with msi_attrib.is_virtual which ensures
> >> functions that program the MSI-X table don't try to write past the end
> >> of the hardware's table.
> > 
> > AFAICT what you've described is what Intel is calling IMS in other
> > contexts.
> > 
> > IMS is fundamentally a way to control MSI interrupt descriptors that
> > are not accessed through PCI SIG compliant means. In this case the NTB
> > driver has to do its magic to relay the addr/data pairs to the real
> > MSI storage in the hidden devices.
> 
> With current applications, it isn't that there is real "MSI storage"
> anywhere; the device on the other side of the bridge is always another
> Linux host which holds the address (or rather mw offset) and data in
> memory to use when it needs to trigger the interrupt of the other
> machine. 

Sure, that is fine "MSI Storage". The triggering device only needs to
store the addr/data pair someplace to be "MSI Storage".

Jason