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From: Jason Gunthorpe <jgg@nvidia.com>
To: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Dey, Megha" <megha.dey@intel.com>, Marc Zyngier <maz@kernel.org>,
	"Jiang, Dave" <dave.jiang@intel.com>,
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	"Raj, Ashok" <ashok.raj@intel.com>,
	"Liu, Yi L" <yi.l.liu@intel.com>,
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	"Ortiz, Samuel" <samuel.ortiz@intel.com>,
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Subject: Re: [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI irq domain
Date: Fri, 7 Aug 2020 10:34:28 -0300
Message-ID: <20200807133428.GT16789@nvidia.com> (raw)
In-Reply-To: <20200807123831.GA645281@kroah.com>

On Fri, Aug 07, 2020 at 02:38:31PM +0200, gregkh@linuxfoundation.org wrote:
> 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 :(

Yes, it is more scalable. The problem with MSI-X is you need actual
physical silicon for each and every vector. This really limits the
number of vectors.

Placing the vector metadata with the queue means it can potentially
live in system memory which is significantly more scalable.

setup/mask/unmask will be slower. The driver might have
complexity. They are not performance path, right?

I don't think it is wrong or right. IMHO the current design where the
addr/data is hidden inside the platform is an artifact of x86's
compatibility legacy back when there was no such thing as message
interrupts. 

If you were starting from a green field I don't think a design would
include the IOAPIC/MSI/MSI-X indirection tables.

> 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?

Intel's SIOV document is an interesting place to start:

https://software.intel.com/content/www/us/en/develop/download/intel-scalable-io-virtualization-technical-specification.html

Though it is more of a rational and a cookbook on how to combine
existing technology pieces. (eg PASID, platform_msi, etc)

The basic approach of SIOV's IMS is that there is no longer a generic
interrupt indirection from numbers to addr/data pairs like
IOAPIC/MSI/MSI-X owned by the common OS code.

Instead the driver itself is responsible to set the addr/data pair
into the device in a device specific way, deal with masking, etc.

This lets the device use an implementation that is not limited by the
harsh MSI-X semantics.

In Linux we already have 'IMS' it is called platform_msi and a few
embedded drivers already work like this. The idea here is to bring it
to PCI.

Jason

  reply index

Thread overview: 97+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-21 16:02 [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 01/18] platform-msi: Introduce platform_msi_ops Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI irq domain Dave Jiang
2020-07-21 16:13   ` Jason Gunthorpe
2020-07-22 16:50     ` Dey, Megha
2020-07-22 18:52   ` Marc Zyngier
2020-07-22 19:59     ` Jason Gunthorpe
2020-07-23  8:51       ` Marc Zyngier
2020-07-24  0:16         ` Jason Gunthorpe
2020-07-24  0:36           ` Thomas Gleixner
2020-08-05 19:18       ` Dey, Megha
2020-08-05 22:15         ` Jason Gunthorpe
2020-08-05 22:36           ` Dey, Megha
2020-08-05 22:53             ` Jason Gunthorpe
2020-08-06  0:13               ` Dey, Megha
2020-08-06  0:19                 ` Jason Gunthorpe
2020-08-06  0:32                   ` Dey, Megha
2020-08-06  0:46                     ` Jason Gunthorpe
2020-08-06 17:10                     ` Thomas Gleixner
2020-08-06 17:58                       ` Dey, Megha
2020-08-06 20:21                         ` Thomas Gleixner
2020-08-06 22:27                           ` Dey, Megha
2020-08-07  8:48                             ` Thomas Gleixner
2020-08-07 12:06                           ` Jason Gunthorpe
2020-08-07 12:38                             ` gregkh
2020-08-07 13:34                               ` Jason Gunthorpe [this message]
2020-08-07 16:47                                 ` Thomas Gleixner
2020-08-07 17:54                                   ` Dey, Megha
2020-08-07 18:39                                     ` Jason Gunthorpe
2020-08-07 20:31                                       ` Dey, Megha
2020-08-08 19:47                                     ` Thomas Gleixner
2020-08-10 21:46                                       ` Thomas Gleixner
2020-08-11  9:53                                         ` Thomas Gleixner
2020-08-11 18:46                                           ` Dey, Megha
2020-08-11 21:25                                             ` Thomas Gleixner
2020-08-11 18:39                                       ` Dey, Megha
2020-08-11 22:39                                         ` Thomas Gleixner
2020-08-07 15:22                             ` Thomas Gleixner
2020-08-05 18:55     ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 03/18] irq/dev-msi: Create IR-DEV-MSI " Dave Jiang
2020-07-21 16:21   ` Jason Gunthorpe
2020-07-22 17:03     ` Dey, Megha
2020-07-22 17:33       ` Jason Gunthorpe
2020-07-22 20:44   ` Thomas Gleixner
2020-08-05 19:02     ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 04/18] irq/dev-msi: Introduce APIs to allocate/free dev-msi interrupts Dave Jiang
2020-07-21 16:25   ` Jason Gunthorpe
2020-07-22 17:05     ` Dey, Megha
2020-07-22 17:35       ` Jason Gunthorpe
2020-08-05 20:19         ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 05/18] dmaengine: idxd: add support for readonly config devices Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 06/18] dmaengine: idxd: add interrupt handle request support Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 07/18] dmaengine: idxd: add DEV-MSI support in base driver Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 08/18] dmaengine: idxd: add device support functions in prep for mdev Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 09/18] dmaengine: idxd: add basic mdev registration and helper functions Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 10/18] dmaengine: idxd: add emulation rw routines Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 11/18] dmaengine: idxd: prep for virtual device commands Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 12/18] dmaengine: idxd: virtual device commands emulation Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 13/18] dmaengine: idxd: ims setup for the vdcm Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 14/18] dmaengine: idxd: add mdev type as a new wq type Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 15/18] dmaengine: idxd: add dedicated wq mdev type Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 16/18] dmaengine: idxd: add new wq state for mdev Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 17/18] dmaengine: idxd: add error notification from host driver to mediated device Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 18/18] dmaengine: idxd: add ABI documentation for mediated device support Dave Jiang
2020-07-21 16:28 ` [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver Greg KH
2020-07-21 17:17   ` Dave Jiang
2020-07-21 21:35   ` Dan Williams
2020-07-21 16:45 ` Jason Gunthorpe
2020-07-21 18:00   ` Dave Jiang
2020-07-22 17:31     ` Dey, Megha
2020-07-22 18:16       ` Jason Gunthorpe
2020-07-21 23:54   ` Tian, Kevin
2020-07-24  0:19     ` Jason Gunthorpe
2020-08-06  1:22       ` Alex Williamson
2020-08-07 12:19         ` Jason Gunthorpe
2020-08-10  7:32           ` Tian, Kevin
2020-08-11 17:00             ` Alex Williamson
2020-08-12  1:58               ` Tian, Kevin
2020-08-12  2:36                 ` Alex Williamson
2020-08-12  3:35                   ` Tian, Kevin
2020-08-12  3:28             ` Jason Wang
2020-08-12  4:05               ` Tian, Kevin
2020-08-13  4:33                 ` Jason Wang
2020-08-13  5:26                   ` Tian, Kevin
2020-08-13  6:01                     ` Jason Wang
2020-08-14 13:23                       ` Jason Gunthorpe
2020-08-17  2:24                         ` Tian, Kevin
2020-08-14 13:35             ` Jason Gunthorpe
2020-08-17  2:12               ` Tian, Kevin
2020-08-18  0:43                 ` Jason Gunthorpe
2020-08-18  1:09                   ` Tian, Kevin
2020-08-18 11:50                     ` Jason Gunthorpe
2020-08-18 16:27                       ` Paolo Bonzini
2020-08-18 16:49                         ` Jason Gunthorpe
2020-08-18 17:05                           ` Paolo Bonzini
2020-08-18 17:18                             ` Jason Gunthorpe
2020-08-19  7:29                       ` Tian, Kevin

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