Re: [PATCH v12 11/29] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command

[Michael Roth <michael.roth@amd.com>]

On Sat, Mar 30, 2024 at 09:31:40PM +0100, Paolo Bonzini wrote:
> On 3/29/24 23:58, Michael Roth wrote:

Cc'ing some more TDX folks.

> > +	memslot = gfn_to_memslot(kvm, params.gfn_start);
> > +	if (!kvm_slot_can_be_private(memslot)) {
> > +		ret = -EINVAL;
> > +		goto out;
> > +	}
> > +
> 
> This can be moved to kvm_gmem_populate.

That does seem nicer, but I hadn't really seen that pattern for
kvm_gmem_get_pfn()/etc. so wasn't sure if that was by design or not. I
suppose in those cases the memslot is already available at the main
KVM page-fault call-sites so maybe it was just unecessary to do the
lookup internally there.

> 
> > +	populate_args.src = u64_to_user_ptr(params.uaddr);
> 
> This is not used if !do_memcpy, and in fact src is redundant with do_memcpy.
> Overall the arguments can be "kvm, gfn, src, npages, post_populate, opaque"
> which are relatively few and do not need the struct.

This was actually a consideration for TDX that was discussed during the
"Finalizing internal guest_memfd APIs for SNP/TDX" PUCK call. In that
case, they have a TDH_MEM_PAGE_ADD seamcall that takes @src and encrypts
it, loads it into the destination page, and then maps it into SecureEPT
through a single call. So in that particular case, @src would be
initialized, but the memcpy() would be unecessary.

It's not actually clear TDX plans to use this interface. In v19 they still
used a KVM MMU hook (set_private_spte) that gets triggered through a call
to KVM_MAP_MEMORY->kvm_mmu_map_tdp_page() prior to starting the guest. But
more recent discussion[1] suggests that KVM_MAP_MEMORY->kvm_mmu_map_tdp_page()
would now only be used to create upper levels of SecureEPT, and the
actual mapping/encrypting of the leaf page would be handled by a
separate TDX-specific interface.

With that model, the potential for using kvm_gmem_populate() seemed
plausible to I was trying to make it immediately usable for that
purpose. But maybe the TDX folks can confirm whether this would be
usable for them or not. (kvm_gmem_populate was introduced here[2] for
reference/background)

-Mike

[1] https://lore.kernel.org/kvm/20240319155349.GE1645738@ls.amr.corp.intel.com/T/#m8580d8e39476be565534d6ff5f5afa295fe8d4f7
[2] https://lore.kernel.org/kvm/20240329212444.395559-3-michael.roth@amd.com/T/#m3aeba660fcc991602820d3703b1265722b871025)


> 
> I'll do that when posting the next version of the patches in kvm-coco-queue.
> 
> Paolo
>