Re: [Xen-devel] [PATCH] Implement rdtscp emulation and rdtscp_aux "support"

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 25.11.09 22:24 >>>
>Xen intentionally does not expose the cpuid rdtscp bit
>to PV OS's or to HVM guests, but PV apps can see this
>bit and, as a result, may choose to use the rdtscp
>instruction.  When a PV guest with such an app is migrated
>to a machine that does not have rdtscp support, the
>app will get killed due to an invalid op.  Fix this
>by emulating the rdtscp instruction.  We also need
>to emulate rdtscp in the case where the machine has
>rdtscp support, but rdtsc emulation is enabled (which
>is unfortunately a different path: a privileged op).

I realize that my comment really applies to your earlier patch introducing
rdtsc emulation, but somehow that aspect slipped my attention back
then: Linux has, as a security option, a mechanism to disable rdtsc for
apps, but your emulation code doesn't honor the guest setting of
CR4.TSD.

Jan


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