Re: [Xen-devel] [PATCH v4 11/26] xen/x86: Improvements to in-hypervisor cpuid sanity checks

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 23, 2016 at 04:36:14PM +0000, 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.
>  * Changing the visibility of RDTSCP based on host TSC stability or virtual
>    TSC mode is bogus, so dropped.

Why is it bogus? It is an PV ABI type CPUID.

Independetly of that you would also need to modify tsc_mode.txt file and all 
uses
of 'tsc_mode=3'.



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