Debug-Registers in HVM domain destroyed

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

Hi,

we've found a problem with debug registers in HVM domains with Xen (we are
running 4.2, but the code in the hypervisor seems to be unchanged in unstable)
on INTEL processors:

Debug registers are restored on vcpu switch only if db7 has any debug events
activated. This leads to problems in the following cases:

- db0-3 are changed by the guest before events are set "active" in db7. In case
   of a vcpu switch between setting db0-3 and db7, db0-3 are lost. BTW: setting
   db7 before db0-3 is no option, as this could trigger debug interrupts due to
   stale db0-3 contents.

- single stepping is used and vcpu switch occurs between the single step trap
   and reading of db6 in the guest. db6 contents (single step indicator) are
   lost in this case.

Any thoughts?


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 62060 2932
Fujitsu                                   e-mail: juergen.gross@ts.fujitsu.com
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