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

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

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.

Roger.


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