Re: [PATCH for-4.8] libxc/x86: Report consistent initial APIC value for PV guests

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

On 11/10/2016 11:12 AM, Jan Beulich wrote:
>>>> On 10.11.16 at 15:55, <andrew.cooper3@citrix.com> wrote:
>> On 10/11/16 14:50, Boris Ostrovsky wrote:
>>> Currently hypervisor provides PV guest's CPUID(1).EBX[31:24] (initial
>>> APIC ID) with contents of that field on the processor that launched
>>> the guest. This results in the guest reporting different initial
>>> APIC IDs across runs.
>>>
>>> We should be consistent in how this value is reported, let's set
>>> it to 0 (which is also what Linux guests expect).
>>>
>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> This surely wants to go along with:
>>
>> andrewcoop@andrewcoop:/local/xen.git/xen$ git diff
>> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
>> index b51b51b..bdf9339 100644
>> --- a/xen/arch/x86/traps.c
>> +++ b/xen/arch/x86/traps.c
>> @@ -985,6 +985,10 @@ void pv_cpuid(struct cpu_user_regs *regs)
>>          uint32_t tmp, _ecx, _ebx;
>>  
>>      case 0x00000001:
>> +        /* Fix up VLAPIC details. */
>> +        b &= 0x00FFFFFFu;
>> +        b |= (curr->vcpu_id * 2) << 24;
>> +
>>          c &= pv_featureset[FEATURESET_1c];
>>          d &= pv_featureset[FEATURESET_1d];
>>  
>>
>> Which brings the PV CPUID handling in line with HVM handling.  Otherwise
>> a guest will see an APIC ID of 0 in all vcpus, which will surely confuse it.
> Which will still result in multiple identical APIC IDs once there are
> 128 or more vCPU-s in a PV guest.


And this change (either with or without *2) makes Linux PV problem of
mismatched APIC[0x20] vs CPUID(1).EBX[31:24] to be back (yes, because
Linux is broken in that it currently makes APIC[0x20] return zero).

You'd obviously be within your rights to say that it's up to Linux to
deal with this but I will ask you to consider taking only the toolstack
part of this for now.


-boris


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