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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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?

> 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.


-boris

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