Dan and All:
Currently
both VTI and non-VTI are using machine TC to emulate guest TR, that is a must
as machine memory a guest get may be discontiguous and may not be proper
aligned. But this introduces another extra issues for guest ld.s.
A guest
ld.s to a TR mapped address will always be success, but in TC emulated TR, it
may fail. Hypervisor is OK to handle that if it get control (i.e tlb miss), but
in some situation, machine side may meet the exception deferral condition that
cause the execution directly get returned with wrong value in target register
(NAT is set). It is probably OK with C compiler generated code as there is chk.s
to recover, but for an assembly wrote code with ld.s especially in guest
kernel, we have problem.
A right
way to deal with that is to let machine side have much tighter deferral
condition than guest side so that HV can get trapped always if it is needed and
handle them. This introduces the DCR virtualization policy. Current non-VTI
approach doesn’t write any guest DCR information
to machine side, we are just lucky now, while VTI approach is directly writing to
machine DCR (dcr.pp is virtualized).
In order to support this, DCR needs to be virtualized,
to minimize this, only dcr.dm bit needs to be virtualized, i.e machine DCR.dm=0
while all other bits are same with guest (dcr.pp is another one need to be
virtualized).
Any
comments?
Eddie
