linux-kernel - Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu

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Date:	Wed, 29 Jun 2016 18:19:27 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	<xen-devel@...ts.xenproject.org>, Juergen Gross <jgross@...e.com>,
	"Stefano Stabellini" <sstabellini@...nel.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, "Ingo Molnar" <mingo@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Jan Beulich <JBeulich@...e.com>
Subject: Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Andrew Cooper <andrew.cooper3@...rix.com> writes:
>
>> On 29/06/16 13:16, Vitaly Kuznetsov wrote:
>>> Andrew Cooper <andrew.cooper3@...rix.com> 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.
>
> So can we rely on ACPI data? Especially on MADT and processor ids there?
> I think we can always guarantee that processor ids there match vCPU
> ids. If yes I can try saving this data when we parse MADT.
>

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


View attachment "0001-x86-acpi-store-APIC-ids-for-future-usage-use-it-in-x.patch" of type "text/x-patch" (5295 bytes)