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

Paolo Bonzini
2024-04-03T12:52:16Z

On Wed, Apr 3, 2024 at 12:58 AM Isaku Yamahata <isaku.yamahata@intel.com> wrote:
> I think TDX can use it with slight change. Pass vcpu instead of KVM, page pin
> down and mmu_lock. TDX requires non-leaf Secure page tables to be populated
> before adding a leaf. Maybe with the assumption that vcpu doesn't run, GFN->PFN
> relation is stable so that mmu_lock isn't needed? What about punch hole?
>
> The flow would be something like as follows.
>
> - lock slots_lock
>
> - kvm_gmem_populate(vcpu)
> - pin down source page instead of do_memcopy.

Both pinning the source page and the memcpy can be done in the
callback. I think the right thing to do is:

1) eliminate do_memcpy, letting AMD code taking care of
copy_from_user.

2) pass to the callback only gfn/pfn/src, where src is computed as

args->src ? args->src + i * PAGE_SIZE : NULL

If another architecture/vendor needs do_memcpy, they can add
something like kvm_gmem_populate_copy.

> - get pfn with __kvm_gmem_get_pfn()
> - read lock mmu_lock
> - in the post_populate callback
> - lookup tdp mmu page table to check if the table is populated.
> lookup only version of kvm_tdp_mmu_map().
> We need vcpu instead of kvm.

Passing vcpu can be done using the opaque callback argument to
kvm_gmem_populate.

Likewise, the mmu_lock can be taken by the TDX post_populate
callback.

Paolo

> - TDH_MEM_PAGE_ADD
> - read unlock mmu_lock
>
> - unlock slots_lock
>
> Thanks,
>
> > 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)