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

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Date:   Thu, 23 Jul 2020 21:16:06 -0300
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Dave Jiang <dave.jiang@...el.com>, vkoul@...nel.org,
        megha.dey@...el.com, bhelgaas@...gle.com, rafael@...nel.org,
        gregkh@...uxfoundation.org, tglx@...utronix.de, hpa@...or.com,
        alex.williamson@...hat.com, jacob.jun.pan@...el.com,
        ashok.raj@...el.com, yi.l.liu@...el.com, baolu.lu@...el.com,
        kevin.tian@...el.com, sanjay.k.kumar@...el.com,
        tony.luck@...el.com, jing.lin@...el.com, dan.j.williams@...el.com,
        kwankhede@...dia.com, eric.auger@...hat.com, parav@...lanox.com,
        dave.hansen@...el.com, netanelg@...lanox.com, shahafs@...lanox.com,
        yan.y.zhao@...ux.intel.com, pbonzini@...hat.com,
        samuel.ortiz@...el.com, mona.hossain@...el.com,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-pci@...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 Thu, Jul 23, 2020 at 09:51:52AM +0100, Marc Zyngier wrote:

> > IIRC on Intel/AMD at least once a MSI is launched it is not maskable.
> 
> Really? So you can't shut a device with a screaming interrupt,
> for example, should it become otherwise unresponsive?

Well, it used to be like that in the APICv1 days. I suppose modern
interrupt remapping probably changes things.

> > So the model for MSI is always "mask at source". The closest mapping
> > to the Linux IRQ model is to say the end device has a irqchip that
> > encapsulates the ability of the device to generate the MSI in the
> > first place.
> 
> This is an x86'ism, I'm afraid. Systems I deal with can mask any
> interrupt at the interrupt controller level, MSI or not.

Sure. However it feels like a bad practice to leave the source
unmasked and potentially continuing to generate messages if the
intention was to disable the IRQ that was assigned to it - even if the
messages do not result in CPU interrupts they will still consume
system resources.

> > I suppose the motivation to make it explicit is related to vfio using
> > the generic mask/unmask functionality?
> > 
> > Explicit seems better, IMHO.
> 
> If masking at the source is the only way to shut the device up,
> and assuming that the device provides the expected semantics
> (a MSI raised by the device while the interrupt is masked
> isn't lost and gets sent when unmasked), that's fair enough.
> It's just ugly.

It makes sense that the masking should follow the same semantics for
PCI MSI masking.

Jason