linux-kernel - Re: [PATCH v8 25/29] vfio-pci: Add a new VFIO_REGION_TYPE

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Date:   Fri, 7 Jun 2019 13:47:22 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Auger Eric <eric.auger@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     "eric.auger.pro@...il.com" <eric.auger.pro@...il.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "joro@...tes.org" <joro@...tes.org>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>,
        "yi.l.liu@...el.com" <yi.l.liu@...el.com>,
        Will Deacon <Will.Deacon@....com>,
        Robin Murphy <Robin.Murphy@....com>,
        "kevin.tian@...el.com" <kevin.tian@...el.com>,
        "ashok.raj@...el.com" <ashok.raj@...el.com>,
        Marc Zyngier <Marc.Zyngier@....com>,
        "peter.maydell@...aro.org" <peter.maydell@...aro.org>,
        Vincent Stehle <Vincent.Stehle@....com>
Subject: Re: [PATCH v8 25/29] vfio-pci: Add a new VFIO_REGION_TYPE_NESTED
 region type

On 07/06/2019 09:28, Auger Eric wrote:
>>> +static const struct vfio_pci_fault_abi fault_abi_versions[] = {
>>> +	[0] = {
>>> +		.entry_size = sizeof(struct iommu_fault),
>>> +	},
>>> +};
>>> +
>>> +#define NR_FAULT_ABIS ARRAY_SIZE(fault_abi_versions)
>>
>> This looks like it's leading to some dangerous complicated code to
>> support multiple user selected ABIs.  How many ABIs do we plan to
>> support?  The region capability also exposes a type, sub-type, and
>> version.  How much of this could be exposed that way?  ie. if we need
>> to support multiple versions, expose multiple regions.
> 
> This is something that was discussed earlier and suggested by
> Jean-Philippe that we may need to support several versions of the ABI
> (typicallu when adding PRI support).
> Exposing multiple region is an interesting idea and I will explore that
> direction.

At the moment the ABI support errors and PRI. We're considering setting
the fault report structure to 64 or 128 bytes (see "[PATCH v2 2/4]
iommu: Introduce device fault data"). 64-byte allows for 2 additional
fields before we have to introduce a new ABI version, while 128 byte
should last us a while.

But that's for adding new fields to existing fault types. It's probably
a good idea to have different region types in VFIO for different fault
types, since userspace isn't necessarily prepared to deal with them. For
example right now userspace doesn't have a method to complete
recoverable faults, so we can't add them to the queue.

Thanks,
Jean