linux-kernel - Re: [PATCH 5/8] ACPI: add processor driver for Xen virtual CPUs.

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Date:	Fri, 10 Feb 2012 12:18:38 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	ke.yu@...el.com, kevin.tian@...el.com,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	lenb@...nel.org, rjw@...k.pl
Cc:	xen-devel@...ts.xensource.com, jeremy@...p.org, konrad@...nel.org,
	stefan.bader@...onical.com, Ian.Campbell@...rix.com,
	mike.mcclurg@...rix.com, liang.tang@...cle.com
Subject: Re: [PATCH 5/8] ACPI: add processor driver for Xen virtual CPUs.

> +	if (pr->id == -1) {
> +		int device_declaration;
> +		int apic_id = -1;
> +
> +		if (!strcmp(acpi_device_hid(device), ACPI_PROCESSOR_OBJECT_HID))
> +			device_declaration = 0;
> +		else
> +			device_declaration = 1;
> +
> +		apic_id = acpi_get_cpuid(pr->handle,
> +			device_declaration, pr->acpi_id);
> +		if (apic_id == -1) {
> +			/* Processor is not present in MADT table */

So I was struggling to find an easy way to make the cases below (where
VCPU != physical CPU) work with using the driver that iterates over the
'processor' and was mystified to why it would not work, even with this
patchset. Found out that the acpi_get_cpuid does this:


201 #ifdef CONFIG_SMP
202         for_each_possible_cpu(i) {
203                 if (cpu_physical_id(i) == apic_id)
204                         return i;
205         }

and since not-online vCPUs (so dom0_max_vcpus) are not in the "possible"
bitmask, we never get to check line 203 and end up returning -1 for
offline/not-present/not-possible vCPUs.

Which means that we end up here:
> +			return 0;
> +		}
> +

instead of going through the pr->id = 0.

By the end of this, the information that the hypervisor gets is
actually limited to the amount of CPUs that we specified in dom0_max_vcpus=

> +		/*
> +		 * It's possible to have pr->id as '-1' even when it's actually
> +		 * present in MADT table, e.g. due to limiting dom0 max vcpus
> +		 * less than physical present number. In such case we still want
> +		 * to parse ACPI processor object information, so mimic the
> +		 * pr->id to CPU-0. This should be safe because we only care
> +		 * about raw ACPI information, which only relies on pr->acpi_id.
> +		 * For other information relying on pr->id and gathered through
> +		 * SMP function call, it's safe to let them run on CPU-0 since
> +		 * underlying Xen will collect them. Only a valid pr->id can
> +		 * make later invocations forward progress.
> +		 */
> +		pr->id = 0;
> +	}
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