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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.


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