linux-kernel - Re: [PATCH 0/9] Support using MSI interrupts in ntb

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Message-ID: <5f8b1616-a005-7658-978f-16466fcc3886@deltatee.com>
Date:   Thu, 31 Jan 2019 16:41:32 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dave Jiang <dave.jiang@...el.com>, linux-kernel@...r.kernel.org,
        linux-ntb@...glegroups.com, linux-pci@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
        Jon Mason <jdmason@...zu.us>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Allen Hubbe <allenbh@...il.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Eric Pilmore <epilmore@...aio.com>
Subject: Re: [PATCH 0/9] Support using MSI interrupts in ntb_transport

On 2019-01-31 3:46 p.m., Dave Jiang wrote:
> I believe irqbalance writes to the file /proc/irq/N/smp_affinity. So 
> maybe take a look at the code that starts from there and see if it would 
> have any impact on your stuff.

Ok, well on my system I can write to the smp_affinity all day and the
MSI interrupts still work fine.

The MSI code is a bit difficult to trace and audit with all the
different chips and the parent chips which I don't have a good
understanding of. But I can definitely see that it could be possible for
some chips to change the address as smp_affinitiy will eventually
sometimes call msi_domain_set_affinity() which does seem to recompose
the message and write it back to the chip.

So, I could relatively easily add a callback to msi_desc to catch this
and resend the MSI address/data. However, I'm not sure how this is ever
done atomically. It seems like there would be a race while the device
updates its address where old interrupts could be triggered. This race
would be much longer for us when sending this information over the NTB
link. Though, I guess if the only change is that it encodes CPU
information in the address then that would not be an issue. However, I'm
not sure I can say that for certain without a comprehensive
understanding of all the IRQ chips.

Any thoughts on this?

Logan