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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/03/2016 10:05 AM, Roger Pau Monné wrote:
> El 3/2/16 a les 15:30, Boris Ostrovsky ha escrit:
> > On 02/03/2016 03:43 AM, Roger Pau Monné wrote:
> > > El 3/2/16 a les 1:17, Andrew Cooper ha escrit:
> > > > On 02/02/2016 23:30, Boris Ostrovsky wrote:
> > > >
> > > > > I think for now I mostly care about APIC and for that I can use HW
> > > > > CPUID bit (which I believe is cleared for HVMlite guests).
> > > > The APIC bit in cpuid is magic and specified as a fast forward of the
> > > > APICBASE_MSR enable bit.
> > > >
> > > > Therefore, the correct architectural behaviour is for this bit to be
> > > > clear if the local APIC is disabled, or indeed not implemented.
> > > >
> > > > With my maintainers hat on, I will reject any attempt to introduce
> > > > non-architectural behaviour; at the moment I am dealing with the
> > > > stupidity that is the PV XSAVE interface, where broken bugfix piled on
> > > > top of broken bugfix has resulted in a situation where many Linux PV
> > > > guests crash if provided with architecturally correct behaviour of the
> > > > OSXSAVE cpuid bit (yet another magic one).
> > > >
> > > > > The trouble is that I need to present Linux as having APIC (boot code
> > > > > doesn't feel good if !cpu_has_apic) so I'll need to keep no-APIC
> > > > > emulation private to Xen-related code. Which is doable.
> > > I have to do the same for FreeBSD, I have to manually switch the APIC
> > > cpuid bit,
> > How? In config file's 'cpuid' option?
> Ah, no, I fix it inside FreeBSD. The FreeBSD kernel stores the result in
> a local variable, which i fixup when booting under HVMlite:
>
> cpu_feature |= CPUID_APIC;
>
> > > or else FreeBSD refuses to do SMP initialization. IMHO, what
> > > we currently do (no APIC cpuid bit) is correct, and when a local APIC is
> > > available the bit will indeed be enabled.
> > >
> > > > I see two choices.
> > > >
> > > > 1) Require that Linux DMLite guests require a Local APIC, and we allow
> > > > that to be a configured option.  Exposing APIC definitely makes sense
> > > > longer term, because APICV hardware acceleration outperforms the
> > > > hypercall-based method.
> > > This is what I aim to do long term, that is provide an emulated local
> > > APIC. The plan was to then also provide ACPI tables in order to notify
> > > the presence of the local and IO APICs (we are going to need both if we
> > > plan to do pci-passthrough of devices with PCI interrupts). Of course
> > > the APIC cpuid bit will also be enabled in this case.
> > 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.
> IMHO the APIC feature bit has a clear meaning: indicate the presence of
> a local APIC. A Xen PV APIC, or however we want to call it it's not a
> local APIC. I think OSes should fixup the CPUID feature bit if that's
> needed for them to work properly, but fixing it in Xen it's an error.
I have a feeling that I confused you (and possibly Andrew) when I said 
"we provide PV APIC ops". By "we" I meant Linux, not Xen. I.e. 
arch/x86/xen/apic.c.
I do essentially the same thing in Linux as you do in FreeBSD by setting 
features bit with setup_force_cpu_cap(X86_FEATURE_APIC). 
http://lists.xenproject.org/archives/html/xen-devel/2016-02/msg00145.html
-boris


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