Re: [RFC PATCH 0/8] KVM: Prepopulate guest memory API

Sean Christopherson
2024-03-11T23:44:29Z

On Sun, Mar 10, 2024, Michael Roth wrote:
> On Fri, Mar 01, 2024 at 09:28:42AM -0800, isaku.yamahata@intel.com wrote:
> > struct kvm_sev_launch_update_data {
> > __u64 uaddr;
> > __u32 len;
> > };
> >
> > - TDX and measurement
> > The TDX correspondence is TDH.MEM.PAGE.ADD and TDH.MR.EXTEND. TDH.MEM.EXTEND
> > extends its measurement by the page contents.
> > Option 1. Add an additional flag like KVM_MEMORY_MAPPING_FLAG_EXTEND to issue
> > TDH.MEM.EXTEND
> > Option 2. Don't handle extend. Let TDX vendor specific API
> > KVM_EMMORY_ENCRYPT_OP to handle it with the subcommand like
> > KVM_TDX_EXTEND_MEMORY.
>
> For SNP this happens unconditionally via SNP_LAUNCH_UPDATE, and with some
> additional measurements via SNP_LAUNCH_FINISH, and down the road when live
> migration support is added that flow will be a bit different. So
> personally I think it's better to leave separate for now.

+1. The only reason to do EXTEND at the same time as PAGE.ADD would be to
optimize setups that want the measurement to be extended with the contents of a
page immediately after the measurement is extended with the mapping metadata for
said page. And AFAIK, the only reason to prefer that approach is for backwards
compatibility, which is not a concern for KVM. I suppose maaaybe some memory
locality performance benefits, but that seems like a stretch.



And I think this whole conversation is moot, because I don't think there's a need
to do PAGE.ADD during KVM_MAP_MEMORY[*]. If KVM_MAP_MEMORY does only the SEPT.ADD
side of things, then both @source (PAGE.ADD) and the EXTEND flag go away.

> But I'd be hesitant to bake more requirements into this pre-mapping
> interface, it feels like we're already overloading it as is.

Agreed. After being able to think more about this ioctl(), I think KVM_MAP_MEMORY
should be as "pure" of a mapping operation as we can make it. It'd be a little
weird that using KVM_MAP_MEMORY is required for TDX VMs, but not other VMs. But
that's really just a reflection of S-EPT, so it's arguably not even a bad thing.

[*] https://lore.kernel.org/all/Ze-TJh0BBOWm9spT@google.com