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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

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