Re: [PATCH] x86/xen: Fix APIC id mismatch warning on Intel

From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Mohit Gambhir <mohit.gambhir@oracle.com>,
	jgross@suse.com, tglx@linutronix.de, mingo@redhat.com,
	hpa@zytor.com, x86@kernel.org, xen-devel@lists.xenproject.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/xen: Fix APIC id mismatch warning on Intel
Date: Sun, 29 Jan 2017 21:58:25 -0500	[thread overview]
Message-ID: <17a5e82f-2e6c-6d16-7a23-1f26810d8e45@oracle.com> (raw)
In-Reply-To: <fe4a713b-b6c7-bcd2-b5b5-1359532617d6@oracle.com>

On 01/26/2017 01:12 PM, Mohit Gambhir wrote:
> This patch fixes the following warning message seen when booting the
> kernel as Dom0 with Xen on Intel machines.
>
> [0.003000] [Firmware Bug]: CPU1: APIC id mismatch. Firmware: 0 APIC: 1]
>
> The code generating the warning in validate_apic_and_package_id() matches
> cpu_data(cpu).apicid (initialized in init_intel()->
> detect_extended_topology() using cpuid) against the apicid returned from
> xen_apic_read(). Now, xen_apic_read() makes a hypercall to retrieve apicid
> for the boot  cpu but returns 0 otherwise. Hence the warning gets thrown
> for all but the boot cpu.
>
> The idea behind xen_apic_read() returning 0 for apicid is that the
> guests (even Dom0) should not need to know what physical processor their
> vcpus are running on. This is because we currently  do not have topology
> information in Xen and also because xen allows more vcpus than physical
> processors. However, boot cpu's apicid is required for loading
> xen-acpi-processor driver on AMD machines. Look at following patch for
> details:
>
> commit 558daa289a40 ("xen/apic: Return the APIC ID (and version) for CPU
> 0.")
>
> So to get rid of the warning, this patch modifies
> xen_cpu_present_to_apicid() to return cpu_data(cpu).apicid instead of
> calling xen_apic_read().
>
> The warning is not seen on AMD machines because init_amd() populates
> cpu_data(cpu).apicid by calling hard_smp_processor_id()->xen_apic_read()
> as opposed to using apicid from cpuid as is done on Intel machines.
>
> Signed-off-by: Mohit Gambhir <mohit.gambhir@oracle.com>

Applied to for-linus-4.11.

-boris