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    SubjectRe: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to paravirt_legacy
    On Mon, Feb 08, 2016 at 04:53:00PM +0000, Andrew Cooper wrote:
    > 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.

    Most of the paravirt_enabled() checks can be replaced with a hardware_subarch
    check once we modify the xen_start_kernel() to add that, today XEN is unused
    even though it was added eons ago. Part of my work was to remove as many
    paravirt_enabled() checks. I'll reply to Andy's original e-mail now indicating
    which ones I could address, it sounds like with this nugget and some other
    work we might be able to address all.

    I should note, in the future the check for subarch would be an explicit part of
    the x86 early init init routines to avoid further issues but only in between
    x86_64_start_reservations() and setup_arch(). How *early* can such a hypercall
    _page check be *safely* be called? I say safely here as if we're not on Xen are
    we OK to muck around and check the same address space?

    Provided we use the subarch for PV to remove a lot of the paravirt_enabled()
    checks, is HVMLite still OK if it ends up using PC subarch and there not being
    a paravirt_enabled() anyamore? Boris O's HVMLite series added paravirt_enabled
    = 1 for the new HVMLite.

    Luis

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