Re: [RFC] /dev/ioasid uAPI proposal

On Fri, Jun 04, 2021 at 05:40:34PM +0200, Paolo Bonzini wrote:

On 04/06/21 17:26, Alex Williamson wrote:

quoted

Let's make sure the KVM folks are part of this decision; a re-cap for
them, KVM currently automatically enables wbinvd emulation when
potentially non-coherent devices are present which is determined solely
based on the IOMMU's (or platform's, as exposed via the IOMMU) ability
to essentially force no-snoop transactions from a device to be cache
coherent.  This synchronization is triggered via the kvm-vfio device,
where QEMU creates the device and adds/removes vfio group fd
descriptors as an additionally layer to prevent the user from enabling
wbinvd emulation on a whim.

IIRC, this latter association was considered a security/DoS issue to
prevent a malicious guest/userspace from creating a disproportionate
system load.

Where would KVM stand on allowing more direct userspace control of
wbinvd behavior?  Would arbitrary control be acceptable or should we
continue to require it only in association to a device requiring it for
correct operation.

Extending the scenarios where WBINVD is not a nop is not a problem for me.
If possible I wouldn't mind keeping the existing kvm-vfio connection via the
device, if only because then the decision remains in the VFIO camp (whose
judgment I trust more than mine on this kind of issue).

Really the question to answer is what "security proof" do you want
before the wbinvd can be enabled

 1) User has access to a device that can issue no-snoop TLPS
 2) User has access to an IOMMU that can not block no-snoop (today)
 3) Require CAP_SYS_RAW_IO
 4) Anyone

#1 is an improvement because it allows userspace to enable wbinvd and
no-snoop optimizations based on user choice

#2 is where we are today and wbinvd effectively becomes a fixed
platform choice. Userspace has no say

#3 is "there is a problem, but not so serious, root is powerful
   enough to override"

#4 is "there is no problem here"

Jason