RE: [Xen-devel] [PATCH] vmx: last branch recording MSR emulation

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> "Li, Xin B" <xin.b.li@xxxxxxxxx> 09.08.07 14:25 >>>
>We don't want to use this feature currently because _each_
>vmentry/vmexit will have to do additional msr operations.

Then why would hardware supply the feature?

Also, as long as the guest doesn't use the feature, no extra saves/restores
are being needed.

Further, since there's no feature detection mechanism (other than family:model)
for a guest to determine whether to make use of that functionality, a guest
trying to do so would likely mis-behave (most likely crash on the attempt to
write a non-zero value to DebugCtlMSR). Certainly, there are more thing like
this that can happen, but for this specific part I'm about to submit a Linux
patch to make use of the functionality - such kernels would then no longer
work under HVM.

Finally, with LBR registers being used in Xen itself (optionally), you'd expose
hypervisor internal information to HVM's, which is generally considered a
security risk.

Jan


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