Re: Constant Stack Canaries

On Sun, 6 Apr 2025 14:21:26 +0000, Scott Lurndal wrote:
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When the exception (in this case an upcall to a more privileged
regime) occurs, the saved state register/stack word should contain the
prior privilege level.   The hypervisor will know from that whether
the upcall was from the guest OS or a guest Application.
>
Note that on ARM, there are restrictions on upcalls to
more privileged regimes - generally a particular regime
can only upcall the next higher privileged regime, so
the user app can only upcall the GuestOS,  the guest OS can only
upcall the HV and the HV is the only regime that can
upcall the secure monitor.

On Sun, 6 Apr 2025 14:32:43 +0000, Scott Lurndal wrote:

That presumes a shared address space between the privilege
levels - which is common for the OS and user-modes.   It's
not common (or particularly useful[*]) at any other privilege
level.

So, is this dichotomy because::
a) HVs are good enough at virtualizing raw HW that GuestOS
   does not need a lot of paravirtualization to be efficient ??
b) GuestOS does not need "that much paravirtualization" to be
   efficient anyway.
c) the kinds of things GuestOS ask HVs to perform is just not
   enough like the kind of things user asks of GuestOS.
d) User and GuestOS evolved in a time before virtualization
   and simply prefer to exist as it used to be ??