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

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Date:   Tue, 8 Jun 2021 15:44:26 +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 08/06/21 15:15, Jason Gunthorpe wrote:
> On Tue, Jun 08, 2021 at 09:56:09AM +0200, Paolo Bonzini wrote:
> 
>>>> 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.
> 
> This isn't the same thing, this is back to trying to have the kernel
> set policy for userspace.

If you want a userspace policy then there would be three states:

* WBINVD enabled because a WBINVD-enabled VFIO device is attached.

* WBINVD potentially enabled but no WBINVD-enabled VFIO device attached

* WBINVD forcefully disabled

KVM_DEV_VFIO_GROUP_ADD/DEL can still be used to distinguish the first 
two.  Due to backwards compatibility, those two describe the default 
behavior; disabling wbinvd can be done easily with a new sub-ioctl of 
KVM_ENABLE_CAP and doesn't require any security proof.

The meaning of WBINVD-enabled is "won't return -ENXIO for the wbinvd 
ioctl", nothing more nothing less.  If all VFIO devices are going to be 
WBINVD-enabled, then that will reflect on KVM as well, and I won't have 
anything to object if there's consensus on the device assignment side of 
things that the wbinvd ioctl won't ever fail.

Paolo