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

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Date:	Tue, 28 Jun 2016 18:28:31 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Vitaly Kuznetsov <vkuznets@...hat.com>,
	<xen-devel@...ts.xenproject.org>
CC:	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

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.

~Andrew