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

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Date:	Wed, 29 Jun 2016 14:16:02 +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>,
	Julien Grall <julien.grall@...rix.com>,
	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>
Subject: Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

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.

-- 
  Vitaly