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
