Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/06/16 13:16, Vitaly Kuznetsov wrote:
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> writes:
>
>> On 28/06/16 17:47, Vitaly Kuznetsov wrote:
>>> @@ -1808,6 +1822,8 @@ static int xen_hvm_cpu_notify(struct notifier_block 
>>> *self, unsigned long action,
>>>     int cpu = (long)hcpu;
>>>     switch (action) {
>>>     case CPU_UP_PREPARE:
>>> +           /* vLAPIC_ID == Xen's vCPU_ID * 2 for HVM guests */
>>> +           per_cpu(xen_vcpu_id, cpu) = cpu_physical_id(cpu) / 2;
>> Please do not assume or propagate this brokenness.  It is incorrect in
>> the general case, and I will be fixing in the hypervisor in due course.
>>
>> Always read the APIC_ID from the LAPIC, per regular hardware.
> (I'm probbaly missing something important - please bear with me)
>
> The problem here is that I need to get _other_ CPU's id before any code
> is executed on that CPU (or, at least, this is the current state of
> affairs if you look at xen_hvm_cpu_up()) so I can't use CPUID/do MSR
> reads/... The only option I see here is to rely on ACPI (MADT) data
> which is stored in x86_cpu_to_apicid (and that's what cpu_physical_id()
> gives us). MADT also has processor id which connects it to DSDT but I'm
> not sure Linux keeps this data. But this is something fixable I guess.

Hmm yes - that is a tricky issue.

It is not safe or correct to assume that xen_vcpu_id is APICID / 2.

This is currently the case for most modern versions of Xen, but isn't
the case for older versions, and won't be the case in the future when I
(or someone else) fixes topology representation for guests.

For this to work, we need one or more of:

1) to provide the guest a full mapping from APIC_ID to vcpu id at boot time.
2) add a new interface where the guest can explicitly query "what is the
vcpu id for the entity with this APIC_ID".
3) Allow HVM guests to identify a vcpu in a hypercall by APIC_ID.

3 is the cleaner approach, but given that vcpu ids have already leaked
into an HVM domains idea of the world, 1 or 2 is probably a better
ladder to dig us out of this hole.

~Andrew.

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