On 04/06/21 17:50, Jason Gunthorpe wrote:

quoted

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

I don't want a security proof myself; I want to trust VFIO to make the 
right judgment and I'm happy to defer to it (via the KVM-VFIO device).

Given how KVM is just a device driver inside Linux, VMs should be a 
slightly more roundabout way to do stuff that is accessible to bare 
metal; not a way to gain extra privilege.

Paolo

  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"