linux-kernel - Re: [Xen-devel] [PATCH] xen/x86: Update topology map for PV VCPUs

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Message-ID: <cf69a4d9-a9a4-2769-9787-bb4636addb33@citrix.com>
Date:   Thu, 6 Oct 2016 15:12:04 +0100
From:   David Vrabel <david.vrabel@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <david.vrabel@...rix.com>, <jgross@...e.com>
CC:     <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, <andrew.cooper3@...rix.com>
Subject: Re: [Xen-devel] [PATCH] xen/x86: Update topology map for PV VCPUs

On 05/10/16 18:09, Boris Ostrovsky wrote:
> Early during boot topology_update_package_map() computes
> logical_pkg_ids for all present processors.
> 
> Later, when processors are brought up, identify_cpu() updates
> these values based on phys_pkg_id which is a function of
> initial_apicid. On PV guests the latter may point to a
> non-existing node, causing logical_pkg_ids to be set to -1.
> 
> Intel's RAPL uses logical_pkg_id (as topology_logical_package_id())
> to index its arrays and therefore in this case will point to index
> 65535 (since logical_pkg_id is a u16). This could lead to either a
> crash or may actually access random memory location.
> 
> As a workaround, we recompute topology during CPU bringup to reset
> logical_pkg_id to a valid value.
> 
> (The reason for initial_apicid being bogus is because it is
> initial_apicid of the processor from which the guest is launched.
> This value is CPUID(1).EBX[31:24])

Applied to for-linus-4.9, thanks.

David