RE: [Xen-devel] [PATCH] vmx: fix debugctl handling

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> "Shan, Haitao" <haitao.shan@xxxxxxxxx> 17.03.08 07:29 >>>
>Hi, Jan,
>
>For IA32_DEBUGCTRL:
>You can use VMX's generic MSR save/load mechanism for loading host value
>for this MSR. Otherwise, the value in this MSR remains unchanged across
>VMX transitions between root and non-root mode.
>I would rather think that this dedicate MSR field is an optimization in
>addition to the generic save/load mechanism. There may not be needs for
>a dedicate host field to make them in pairs.

The point I'm trying to make is that the hypervisor has no way of
allowing the guest to modify e.g. the DS area control bits in this
register without adding a host load MSR; even for the BTF and LBR
bits it seems suspicious to expect the hypervisor to tolerate the
guest register settings. Basically, whenever the guest is permitted
to set the MSR to a non-zero value the host will need to load the
MSR during vmexit, and hence requiring explicit adding of the MSR
to the set that is loaded through the generic mechanism seems
inconsistent.

But nevertheless thanks for clarifying things work the way they
appear to based solely on the VMCS documentation, and namely
that there's no way to avoid the explicit MSR load.

Jan


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