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

On Thu, Apr 07, 2016 at 12:57:11PM +0100, Andrew Cooper 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.

Oracle added this in so I am going to do some homework to see if it
was ever used and such. If it was.. then we will have to treat it as
some regression fix or maybe add some more duct-tape if tsc_mode=3 is enabled
in the guest config. <sigh>

>  * 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: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

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