RE: [RFC] /dev/ioasid uAPI proposal

[Tian, Kevin]

> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Sent: Wednesday, October 27, 2021 6:32 PM
> 
> 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.

Thanks for confirming it!

> 
> > 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?".
> 

Agree. It's always good to keep such mindset in thought practice.

Thanks
Kevin