Messages in this thread | | | Date | Tue, 9 Feb 2016 07:22:37 +0100 | From | "Luis R. Rodriguez" <> | Subject | Re: [Xen-devel] [PATCH v2 3/3] paravirt: rename paravirt_enabled to paravirt_legacy |
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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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