linux-kernel - Re: [PATCH RFC 00/15] Add VFIO mediated device support and IMS support for the idxd driver.

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Date:   Fri, 1 May 2020 15:32:35 -0700
From:   "Dey, Megha" <megha.dey@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Dave Jiang <dave.jiang@...el.com>, vkoul@...nel.org,
        maz@...nel.org, 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,
        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 00/15] Add VFIO mediated device support and IMS
 support for the idxd driver.



On 4/23/2020 12:44 PM, Jason Gunthorpe wrote:
>>>> The mdev utilizes Interrupt Message Store or IMS[3] instead of MSIX for
>>>> interrupts for the guest. This preserves MSIX for host usages and also allows a
>>>> significantly larger number of interrupt vectors for guest usage.
>>>
>>> I never did get a reply to my earlier remarks on the IMS patches.
>>>
>>> The concept of a device specific addr/data table format for MSI is not
>>> Intel specific. This should be general code. We have a device that can
>>> use this kind of kernel capability today.
>>
>> I am sorry if I did not address your comments earlier.
> 
> It appears noboy from Intel bothered to answer anyone else on that RFC
> thread:
> 
> https://lore.kernel.org/lkml/1568338328-22458-1-git-send-email-megha.dey@linux.intel.com/
> 
> However, it seems kind of moot as I see now that this verion of IMS
> bears almost no resemblance to the original RFC.

hmm yeah, we changed most of the code after getting a lot of feedback 
from you and folks at plumbers. But yes, I should have replied to all 
the feedback, lesson learnt :)

> 
> That said, the similiarity to platform-msi was striking, does this new
> version harmonize with that?

yes!
> 
>> The present IMS code is quite generic, most of the code is in the drivers/
>> folder. We basically introduce 2 APIS: allocate and free IMS interrupts and
>> a IMS IRQ domain to allocate these interrupts from. These APIs are
>> architecture agnostic.
>>
>> We also introduce a new IMS IRQ domain which is architecture specific. This
>> is because IMS generates interrupts only in the remappable format, hence
>> interrupt remapping should be enabled for IMS. Currently, the interrupt
>> remapping code is only available for Intel and AMD and I don’t see anything
>> for ARM.
> 
> I don't understand these remarks though - IMS is simply the mapping of
> a MemWr addr/data pair to a Linux IRQ number? Why does this intersect
> with remapping?
> 

 From your comments so far, I think your requirement is a subset of what 
IMS is trying to do.

What you want:
have a dynamic means of allocating platform-msi interrupts

On top of this IMS has a requirement that all of the interrupts should 
be remapped.

So we can have tiered code: generic dynamic platform-msi infrastructure
and add the IMS specific bits (Intel specific) on top of this.

The generic code will have no reference to IMS.

> AFAIK, any platform that supports MSI today should have the inherent
> HW capability to support IMS.
> 
>> Also, could you give more details on the device that could use IMS? Do you
>> have some driver code already? We could then see if and how the current IMS
>> code could be made more generic.
> 
> We have several devices of interest, our NICs have very flexible PCI,
> so it is no problem to take the MemWR addr/data from someplace other
> than the MSI tables.
> 
> For this we want to have some way to allocate Linux IRQs dynamically
> and get a addr/data pair to trigger them.
> 
> Our NIC devices are also linked to our ARM SOC family, so I'd expect
> our ARM's to also be able to provide these APIs as the platform.

cool, so I will hope that you can test out the generic APIs from the ARM 
side!
> 
> Jason
>