Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map_memory()

[Huang, Kai]

On Tue, 2024-03-12 at 07:20 -0700, Sean Christopherson wrote:
> On Tue, Mar 12, 2024, Kai Huang wrote:
> > > Wait. KVM doesn't *need* to do PAGE.ADD from deep in the MMU.  The only inputs to
> > > PAGE.ADD are the gfn, pfn, tdr (vm), and source.  The S-EPT structures need to be
> > > pre-built, but when they are built is irrelevant, so long as they are in place
> > > before PAGE.ADD.
> > > 
> > > Crazy idea.  For TDX S-EPT, what if KVM_MAP_MEMORY does all of the SEPT.ADD stuff,
> > > which doesn't affect the measurement, and even fills in KVM's copy of the leaf EPTE, 
> > > but tdx_sept_set_private_spte() doesn't do anything if the TD isn't finalized?
> > > 
> > > Then KVM provides a dedicated TDX ioctl(), i.e. what is/was KVM_TDX_INIT_MEM_REGION,
> > > to do PAGE.ADD.  KVM_TDX_INIT_MEM_REGION wouldn't need to map anything, it would
> > > simply need to verify that the pfn from guest_memfd() is the same as what's in
> > > the TDP MMU.
> > 
> > One small question:
> > 
> > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
> > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
> > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
> > return error when it finds the region hasn't been pre-populated?
> 
> Return an error.  I don't love the idea of bleeding so many TDX details into
> userspace, but I'm pretty sure that ship sailed a long, long time ago.

In this case, IIUC the KVM_MAP_MEMORY ioctl() will be mandatory for TDX
(presumbly also SNP) guests, but _optional_ for other VMs.  Not sure whether
this is ideal.

And just want to make sure I understand the background correctly:

The KVM_MAP_MEMORY ioctl() is supposed to be generic, and it should be able to
be used by any VM but not just CoCo VMs (including SW_PROTECTED ones)?

But it is only supposed to be used by the VMs which use guest_memfd()?  Because
IIUC for normal VMs using mmap() we already have MAP_POPULATE for this purpose.

Looking at [*], it doesn't say what kind of VM the sender was trying to use.

Therefore can we interpret KVM_MAP_MEMORY ioctl() is effectively for CoCo VMs? 
SW_PROTECTED VMs can also use guest_memfd(), but I believe nobody is going to
use it seriously.

[*] https://lore.kernel.org/all/65262e67-7885-971a-896d-ad9c0a760907@xxxxxxxxx/