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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.11.16 at 15:55, <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> 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.

Jan


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