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

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Date:   Wed, 27 Oct 2021 12:32:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        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 27/10/21 08:18, Tian, Kevin wrote:
>> I absolutely do *not* want an API that tells KVM to enable WBINVD.  This
>> is not up for discussion.
>>
>> But really, let's stop calling the file descriptor a security proof or a
>> capability.  It's overkill; all that we are doing here is kernel
>> acceleration of the WBINVD ioctl.
>>
>> As a thought experiment, let's consider what would happen if wbinvd
>> caused an unconditional exit from guest to userspace.  Userspace would
>> react by invoking the ioctl on the ioasid.  The proposed functionality
>> is just an acceleration of this same thing, avoiding the
>> guest->KVM->userspace->IOASID->wbinvd trip.
> 
> While the concept here makes sense, in reality implementing a wbinvd
> ioctl for userspace requiring iommufd (previous /dev/ioasid is renamed
> to /dev/iommu now) to track dirty CPUs that a given process has been
> running since wbinvd only flushes local cache.
> 
> Is it ok to omit the actual wbinvd ioctl here and just leverage vfio-kvm
> contract to manage whether guest wbinvd is emulated as no-op?

Yes, it'd be okay for me.  As I wrote in the message, the concept of a 
wbinvd ioctl is mostly important as a thought experiment for what is 
security sensitive and what is not.  If a wbinvd ioctl would not be 
privileged on the iommufd, then WBINVD is not considered privileged in a 
guest either.

That does not imply a requirement to implement the wbinvd ioctl, though. 
Of course, non-KVM usage of iommufd systems/devices with non-coherent 
DMA would be less useful; but that's already the case for VFIO.

> btw does kvm community set a strict criteria that any operation that
> the guest can do must be first carried in host uAPI first? In concept
> KVM deals with ISA-level to cover both guest kernel and guest user
> while host uAPI is only for host user. Introducing new uAPIs to allow
> host user doing whatever guest kernel can do sounds ideal, but not
> exactly necessary imho.

I agree; however, it's the right mindset in my opinion because 
virtualization (in a perfect world) should not be a way to give 
processes privilege to do something that they cannot do.  If it does, 
it's usually a good idea to ask yourself "should this functionality be 
accessible outside KVM too?".

Thanks,

Paolo