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

On 03/02/16 14:30, Boris Ostrovsky wrote:
> 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.

PV guests currently have Xen's APIC leaked through despite not having
access to an APIC.  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.


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