From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bandan Das Subject: Re: APIC_ID in apic_reg_write() Date: Wed, 29 Apr 2015 14:54:50 -0400 Message-ID: References: <55412433.7080805@siemens.com> Mime-Version: 1.0 Content-Type: text/plain Cc: kvm@vger.kernel.org To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43549 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751601AbbD2Syz (ORCPT ); Wed, 29 Apr 2015 14:54:55 -0400 In-Reply-To: <55412433.7080805@siemens.com> (Jan Kiszka's message of "Wed, 29 Apr 2015 20:34:27 +0200") Sender: kvm-owner@vger.kernel.org List-ID: Jan Kiszka writes: > Am 2015-04-29 um 18:47 schrieb Bandan Das: >> >> Why do we allow writes to APIC_ID ? On all _newer_ processors, it's >> read only. The spec doesn't explicitly mention it though, or atleast >> I couldn't find it. Does userspace have a reason to modify it ? > > The APIC ID is read-only for x2APIC. It remains R/W for xAPIC. Are you sure ? In 10.4 of the SDM, there is Note that says "In processors based on Intel microarchitecture code name Nehalem the Local APIC ID Register is no longer Read/Write; it is Read Only." And I can verify on a SandyBridge and Haswell system that it's RO there too. In fact, that was one of the reasons I had submitted a patch to remove verify_local_APIC() from x86/kernel/apic.c (4399c03c678) If I am wrong we need to revert atleast the associated commit message :) > Jan