[Xen-devel] [PATCH] Bug in HVM rtc emulation: wrongly placed return statement in rtc_ioport_write. Also comment on possible flaw in current io intercept logic.

["Trolle Selander" <trolle.selander@xxxxxxxxx>]

The rtc emulation currently has a mis-nested return statement, causing rtc_ioport_write to return 0 instead of 1 after doing an rtc cmos memory write.

This also exposed what looks to me like a fairly flaw in the io intercept handling; currently, if there's a handler set up, hvm_io_intercept returns the return value of the call to that handler, meaning that a handler that runs but returns 0 will get interpreted the same as an io which has no handler (and thus should be handled in qemu-dm rather than the hypervisor). This means that the io gets passed on to hvm_send_assist_req (in the case of the rtc bug this happened even though it had already been successfully handled) and ends up in qemu-dm.
I'm not sure if this is merely a "thinko", or if it's designed this way to have io ranges that are "partially" handled in the hypervisor and partially in qemu. Even if it's the latter, I think there needs to be a mechanism in place to distinguish the failure of a handler for an intercepted io from the nonexistence of a handler.

/Trolle

Attachment: rtc_return_nesting_fix.patch
Description: Binary data

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