quoted

quoted

This isn't right, vendor_hyp_bmap is very much load bearing. We have a
documented UAPI that allows userspace to control the hypercalls exposed
to the guest.

The idea being a user wants kernel rollback safety and doesn't expose
hypercalls that could potentially be revoked.

https://docs.kernel.org/virt/kvm/arm/fw-pseudo-registers.html#bitmap-

feature-firmware-registers

To add:

KVM cannot advertise the DISCOVER_IMPL* stuff unconditionally, since the
expectation is that userspace implements these hypercalls. These bits
may need to be writable from userspace but have a reset value of 0.

Ok. So IIUC, vendor_hyp_bmap actually holds the information which Vendor Hyp
services  are available to the user space and can be get/set using GET/SET _ONE_REG
interfaces.

Right.

Currently this bitmap is a 64 bit one and if we have to have a one to one mapping
between these bitmap and the hypercall function numbers, then that requires
some changes.

Because function numbers 2-63 are now reserved for pKVM and the new ones
introduced in this series take 64 & 65.

May be we can have KVM_REG_ARM_VENDOR_HYP_BMAP_2  which represents
64-127?

Or can we take the next available bits(2 & 3) for KVM_REG_ARM_VENDOR_HYP_BMAP
and then map it to the function number appropriately (64 & 65) when
ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID is handled?

Thoughts?

Adding a second register seems reasonable so we can preserve the mapping
of bit position / hypercall #.

-- 
Thanks,
Oliver