linux-kernel - Re: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV

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Date:   Fri, 7 Aug 2020 14:38:31 +0200
From:   "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Dey, Megha" <megha.dey@...el.com>, Marc Zyngier <maz@...nel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Lu, Baolu" <baolu.lu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Lin, Jing" <jing.lin@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "netanelg@...lanox.com" <netanelg@...lanox.com>,
        "shahafs@...lanox.com" <shahafs@...lanox.com>,
        "yan.y.zhao@...ux.intel.com" <yan.y.zhao@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Ortiz, Samuel" <samuel.ortiz@...el.com>,
        "Hossain, Mona" <mona.hossain@...el.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI
 irq domain

On Fri, Aug 07, 2020 at 09:06:50AM -0300, Jason Gunthorpe wrote:
> On Thu, Aug 06, 2020 at 10:21:11PM +0200, Thomas Gleixner wrote:
> 
> > Optionally? Please tell the hardware folks to make this mandatory. We
> > have enough pain with non maskable MSI interrupts already so introducing
> > yet another non maskable interrupt trainwreck is not an option.
> 
> Can you elaborate on the flows where Linux will need to trigger
> masking?
> 
> I expect that masking will be available in our NIC HW too - but it
> will require a spin loop if masking has to be done in an atomic
> context.
> 
> > It's more than a decade now that I tell HW people not to repeat the
> > non-maskable MSI failure, but obviously they still think that
> > non-maskable interrupts are a brilliant idea. I know that HW folks
> > believe that everything they omit can be fixed in software, but they
> > have to finally understand that this particular issue _cannot_ be fixed
> > at all.
> 
> Sure, the CPU should always be able to shut off an interrupt!
> 
> Maybe explaining the goals would help understand the HW perspective.
> 
> Today HW can process > 100k queues of work at once. Interrupt delivery
> works by having a MSI index in each queue's metadata and the interrupt
> indirects through a MSI-X table on-chip which has the
> addr/data/mask/etc.
> 
> What IMS proposes is that the interrupt data can move into the queue
> meta data (which is not required to be on-chip), eg along side the
> producer/consumer pointers, and the central MSI-X table is not
> needed. This is necessary because the PCI spec has very harsh design
> requirements for a MSI-X table that make scaling it prohibitive.
> 
> So an IRQ can be silenced by deleting or stopping the queue(s)
> triggering it. It can be masked by including masking in the queue
> metadata. We can detect pending by checking the producer/consumer
> values.
> 
> However synchronizing all the HW and all the state is now more
> complicated than just writing a mask bit via MMIO to an on-die memory.

Because doing all of the work that used to be done in HW in software is
so much faster and scalable?  Feels really wrong to me :(

Do you all have a pointer to the spec for this newly proposed stuff
anywhere to try to figure out how the HW wants this to all work?

thanks,

greg k-h