Re: [Xen-devel] [PATCH] x86/hvm: Provide list of emulated features in HVM CPUID leaf

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/03/2016 09:37 AM, Andrew Cooper wrote:
> On 03/02/16 14:30, Boris Ostrovsky wrote:
> > One might say that in Linux we have APIC even for PV guests
> > --- we provide PV APIC ops. That's what I am using as justification
> > for stating that the HVMlite guest has APIC to force-set
> > X86_FEATURE_APIC bit. So this is somewhat similar to what Andrew is
> > proposing in his option#2 (quoted below for convenience):
> >
> >      2) Find a way of telling the Linux boot path "trust me - here is
> > an APIC
> >      driver - dont go looking under the hood".  Possibly by registering a
> >      cpuid pvop which re-inserts the APIC bit, although this is liable to
> >      cause the boot code to then inspect the APICBASE_MSR, which will
> > cause
> >      it to blow up slightly later on.
> PV guests currently have Xen's APIC leaked through despite not having
> access to an APIC.
What do you mean by "leaked through"?

> As with the XSAVE leakage, this has become an
> defacto part of the ABI despite being architecturally wrong.
>
> I expect PVOps Linux will blow up when run on older hardware which does
> lack a real APIC, or one which is disabled in the BIOS.
I didn't mean to say that we set X86_FEATURE_APIC for PV guests, it's 
only done for HVMlite.
I don't think there is hardware with VT/SVM that doesn't have APIC. 
Besides, in Linux we have
    config XEN_PVHVM
        def_bool y
        depends on XEN && PCI && X86_LOCAL_APIC

and HVMlite is considered PVHVM.

-boris



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel