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Date: Thu, 30 Jun 2016 03:10:11 -0600
From: "Jan Beulich" <JBeulich@suse.com>
To: "Andrew Cooper" <andrew.cooper3@citrix.com>,
        "Vitaly Kuznetsov" <vkuznets@redhat.com>
Cc: "David Vrabel" <david.vrabel@citrix.com>,
        "Stefano Stabellini" <sstabellini@kernel.org>, <x86@kernel.org>,
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        <xen-devel@lists.xenproject.org>,
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        "Ingo Molnar" <mingo@redhat.com>, "Juergen Gross" <JGross@suse.com>,
        <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id
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>>> On 29.06.16 at 18:27, <andrew.cooper3@citrix.com> wrote:
> On 29/06/16 17:19, Vitaly Kuznetsov wrote:
>> To explain better what I'm trying to suggest here please take a look at
>> the attached patch. If we can guarantee long term that ACPI id always
>> equals to Xen's idea of vCPU id this is probably the easiest way.
>>
>> -- Vitaly
> 
> The code in hvmloader which sets up the MADT does:
> 
>     for ( i = 0; i < hvm_info->nr_vcpus; i++ )
>     {
>         memset(lapic, 0, sizeof(*lapic));
>         lapic->type    = ACPI_PROCESSOR_LOCAL_APIC;
>         lapic->length  = sizeof(*lapic);
>         /* Processor ID must match processor-object IDs in the DSDT. */
>         lapic->acpi_processor_id = i;
>         lapic->apic_id = LAPIC_ID(i);
>         lapic->flags = (test_bit(i, hvm_info->vcpu_online)
>                         ? ACPI_LOCAL_APIC_ENABLED : 0);
>         lapic++;
>     }
> 
> So relying on the acpi_processor_id does look to be reliable.  That code
> hasn't changed since 2007, and that was only a bugfix.  I would go so
> far as to say it is reasonable for us to guarantee this in the guest ABI.

In fact - is there any other way a guest could learn the vCPU IDs
of its CPUs in a reliable way? I don't think so, and hence this de
facto already is part of the ABI; we should of course spell it out
somewhere.

Jan