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Re: [Xen-devel] [PATCH v5 06/21] xen/x86: Improvements to in-hypervisor cpuid sanity checks

>>> On 07.04.16 at 13:57, <andrew.cooper3@xxxxxxxxxx> wrote:
> Currently, {pv,hvm}_cpuid() has a large quantity of essentially-static logic
> for modifying the features visible to a guest.  A lot of this can be 
> subsumed
> by {pv,hvm}_featuremask, which identify the features available on this
> hardware which could be given to a PV or HVM guest.
> This is a step in the direction of full per-domain cpuid policies, but lots
> more development is needed for that.  As a result, the static checks are
> simplified, but the dynamic checks need to remain for now.
> As a side effect, some of the logic for special features can be improved.
> OSXSAVE and OSPKE will be automatically cleared because of being absent in 
> the
> featuremask.  This allows the fast-forward logic to be more simple.
> In addition, there are some corrections to the existing logic:
>  * Hiding PSE36 out of PAE mode is architecturally wrong.  It turns out that
>    it was a bugfix for running HyperV under Xen, which wanted to see PSE36
>    even after choosing to use PAE paging.  PSE36 is not supported by shadow
>    paging, so is hidden from non-HAP guests, but is still visible for HAP
>    guests.  It is also leaked into non-HAP guests when the guest is already
>    running in PAE mode.
>  * Changing the visibility of RDTSCP based on host TSC stability or virtual
>    TSC mode is bogus, so dropped.
>  * When emulating Intel to a guest, the common features in e1d should be
>    cleared.
>  * The APIC bit in e1d (on non-Intel) is also a fast-forward from the
> As a small improvement, use compiler-visible &'s and |'s, rather than
> {clear,set}_bit().
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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