Re: [RFC] /dev/ioasid uAPI proposal

Paolo Bonzini
2021-10-27T10:32:35Z

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