[Xen-devel] Nested events in 64bit mini-OS

[Xu Zhang <zhxwhu@xxxxxxxxx>]

Dear all,

I don't understand why 64-bit mini-OS doesn't check against nested Xen 
events (in entry.S). In 32-bit code, a critical section is defined and 
re-entrant is checked. A fix-up routine is executed to coalesce the 
stack frames if that's the case, because there is a window for nested 
events to happen after re-enabling event delivery and before a direct iret.
In 64-bit mini-OS, however, a critical section is defined but not used. 
My understanding is that ideally, one could
a) unmask event and do an direct iret, but check against nested events, 
try to fix stack frames if that happens; or
b) do not re-enable event and use hypercall iret to return (no need to 
fix-up stack frames).
64-bit mini-OS seems to adopt a mixed use of both (in HYPERVISOR_IRET). 
mini-OS doesn't have an userspace, so unless an NMI happened, it always 
perform interrupt/exception return with machine instruction iret, 
without checking against nested events. This is wrong to me. Am I 
missing something here?
Please advise.

Thank you,
Xu

--
xu

:q!


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