Re: [PATCH v6] hw/misc/vmfwupdate: Introduce hypervisor fw-cfg interface support

["Jörg Rödel" <joro@8bytes.org>]

Hi Ani,

Don't get me wrong, I really like the general idea of vmfwupdate as
a way to implement BYOFW, and for non-confidential VMs it is great. I
just think the interface design is not well suited for confidential VMs
yet and want to discuss how to change that.

On Thu, Mar 13, 2025 at 04:02:11PM +0530, Ani Sinha wrote:
> Forgive my ignorance, today none of the hyperscalers use IGVM to
> define this. How then the expected launch measurement is derived?

There is at least one Hyperscaler which uses IGVM, another implemented
support for it in their hypervisor (although not for a BYOFW feature,
yet). In my view this problem is still widely being worked on today by
the CSPs. If you are lucky, and since the firmware is provided by the
CSP as well, they publish expected launch measurments for each of their
coco platforms.

> Our vmfwupdate interface does not make any attempt to define this in
> any way or make this an ABI. Its intentionally open ended. It's really
> up to the guest to set this up any way they wish. This is the same
> when the guest provides the igvm file that defines the above.

Without a way to supply user-defined setup steps of a confidential VM
the host-VMM has to hardcode them. And that hardcoded behavior becomes
ABI implicitly, because any change to this behavior will also change the
actual launch measurements, potentially breaking existing users.

This happened in the past already when KVM decided to hardcode a
different set of SEV-Features by enabling Debug-Swap. I strongly believe
that with KVM/QEMU we should work in a direction which makes those
breakages unlikely in the future.

Regards,

	Joerg