Re: [PATCH] ptp: Add vDSO-style vmclock support

On Thu, 2024-07-25 at 01:54 -0400, Michael S. Tsirkin wrote:

one other thing worth mentioning is that this design can't work
with confidential computing setups. By comparison, mapping e.g. a
range in a PCI BAR would work for these setups.

Why so? This is just like mapping a PCI BAR, isn't it? It's cacheable
MMIO space, *not* part of the encrypted guest RAM ranges. It just
happens to be discovered through the _CRS of an ACPI device, not the
BAR of a PCI device.

Is there a reason this functionality is not interesting for
confidential VMs?

It is. In fact, that was one of the reasons for doing it as mappable
MMIO space, instead of having the guest allocate a portion of its own
RAM and invoke a hypervisor enlightenment to populate it. (Although the
latter *can* work with CC too, as demonstrated by e.g. ptp_kvm).