linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Tue, 8 Jun 2021 09:56:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On 07/06/21 19:59, Jason Gunthorpe wrote:
>> The KVM interface is the same kvm-vfio device that exists already.  The
>> userspace API does not need to change at all: adding one VFIO file
>> descriptor with WBINVD enabled to the kvm-vfio device lets the VM use WBINVD
>> functionality (see kvm_vfio_update_coherency).
>
> The problem is we are talking about adding a new /dev/ioasid FD and it
> won't fit into the existing KVM VFIO FD interface. There are lots of
> options here, one is to add new ioctls that specifically use the new
> FD, the other is to somehow use VFIO as a proxy to carry things to the
> /dev/ioasid FD code.

Exactly.

>> Alternatively you can add a KVM_DEV_IOASID_{ADD,DEL} pair of ioctls. But it
>> seems useless complication compared to just using what we have now, at least
>> while VMs only use IOASIDs via VFIO.
>
> The simplest is KVM_ENABLE_WBINVD(<fd security proof>) and be done
> with it.

The simplest one is KVM_DEV_VFIO_GROUP_ADD/DEL, that already exists and 
also covers hot-unplug.  The second simplest one is KVM_DEV_IOASID_ADD/DEL.

It need not be limited to wbinvd support, it's just a generic "let VMs 
do what userspace can do if it has access to this file descriptor". 
That it enables guest WBINVD is an implementation detail.

>> Either way, there should be no policy attached to the add/delete operations.
>> KVM users want to add the VFIO (or IOASID) file descriptors to the device
>> independent of WBINVD.  If userspace wants/needs to apply its own policy on
>> whether to enable WBINVD or not, they can do it on the VFIO/IOASID side:
> 
> Why does KVM need to know abut IOASID's? I don't think it can do
> anything with this general information.

Indeed, it only uses them as the security proofs---either VFIO or IOASID 
file descriptors can be used as such.

Paolo