Re: [PATCH RFC gmem v1 4/8] KVM: x86: Add gmem hook for invalidating memory

[Sean Christopherson <seanjc@google.com>]

On Fri, Feb 09, 2024, Steven Price wrote:
> >> One option that I've considered is to implement a seperate CCA ioctl to
> >> notify KVM whether the memory should be mapped protected.
> > 
> > That's what KVM_SET_MEMORY_ATTRIBUTES+KVM_MEMORY_ATTRIBUTE_PRIVATE is for, no?
> 
> Sorry, I really didn't explain that well. Yes effectively this is the
> attribute flag, but there's corner cases for destruction of the VM. My
> thought was that if the VMM wanted to tear down part of the protected
> range (without making it shared) then a separate ioctl would be needed
> to notify KVM of the unmap.

No new uAPI should be needed, because the only scenario time a benign VMM should
do this is if the guest also knows the memory is being removed, in which case
PUNCH_HOLE will suffice.

> >> This 'solves' the problem nicely except for the case where the VMM
> >> deliberately punches holes in memory which the guest is using.
> > 
> > I don't see what problem there is to solve in this case.  PUNCH_HOLE is destructive,
> > so don't do that.
> 
> A well behaving VMM wouldn't PUNCH_HOLE when the guest is using it, but
> my concern here is a VMM which is trying to break the host. In this case
> either the PUNCH_HOLE needs to fail, or we actually need to recover the
> memory from the guest (effectively killing the guest in the process).

The latter.  IIRC, we talked about this exact case somewhere in the hour-long
rambling discussion on guest_memfd at PUCK[1].  And we've definitely discussed
this multiple times on-list, though I don't know that there is a single thread
that captures the entire plan.

The TL;DR is that gmem will invoke an arch hook for every "struct kvm_gmem"
instance that's attached to a given guest_memfd inode when a page is being fully
removed, i.e. when a page is being freed back to the normal memory pool.  Something
like this proposed SNP patch[2].

Mike, do have WIP patches you can share?

[1] https://drive.google.com/corp/drive/folders/116YTH1h9yBZmjqeJc03cV4_AhSe-VBkc?resourcekey=0-sOGeFEUi60-znJJmZBsTHQ
[2] https://lore.kernel.org/all/20231230172351.574091-30-michael.roth@amd.com