Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to paravirt_legacy

[Andrew Cooper]

On 08/02/16 16:45, Borislav Petkov wrote:
> On Mon, Feb 08, 2016 at 04:38:40PM +0000, Andrew Cooper wrote:
>> Does the early loader have extable support?  If so, this is fairly easy
>> to fix.  If not, we have a problem.
> It doesn't and regardless, you want to have this CPUID querying as
> simple as possible. No special handling, no special prefixes as it
> should be able to run on other hypervisors too.
>
> If one can't execute a simple CPUID(0x4...) on a xen guest and get the
> results back, then for early, we will have to do what we've done until
> now and simply emulate the MSR accesses.
>
> Later code can use then xen_cpuid() and all is fine. We should still get
> rid of paravirt_enabled() though.
>

The force emulation prefix starts with a ud2a instruction, so extable is
to prevent it breaking on non-Xen systems.  However, if extable isn't
available, this point is moot.

As an alternative check which should be doable this early on, peeking in
the head of hypercall_page should work.  If Linux was booted as a PV
guest, the hypercall_page will have been constructed by the domain
builder, and won't have 0x90's in it.

~Andrew