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

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

On 02/03/2016 09:37 AM, Andrew Cooper wrote:
> On 03/02/16 14:30, Boris Ostrovsky wrote:
>>
>> 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.

What do you mean by "leaked through"?

> 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.

I didn't mean to say that we set X86_FEATURE_APIC for PV guests, it's 
only done for HVMlite.

I don't think there is hardware with VT/SVM that doesn't have APIC. 
Besides, in Linux we have

     config XEN_PVHVM
         def_bool y
         depends on XEN && PCI && X86_LOCAL_APIC

and HVMlite is considered PVHVM.

-boris