From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: iommu=0 leading to panic when system defaults to using x2apic Date: Tue, 14 Dec 2010 09:12:20 +0000 Message-ID: <4D0743040200007800027C0F@vpn.id2.novell.com> References: <4D0215E0020000780002734A@vpn.id2.novell.com> <4D0224E602000078000273AD@vpn.id2.novell.com> <749B9D3DBF0F054390025D9EAFF47F22317AA2F4@shsmsx501.ccr.corp.intel.com> <4D0257D7020000780002747B@vpn.id2.novell.com> <987664A83D2D224EAE907B061CE93D530193BB9EB8@orsmsx505.amr.corp.intel.com> <4D07372F0200007800027BBF@vpn.id2.novell.com> <4D072EF3.5000405@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <4D072EF3.5000405@intel.com> Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Weidong Han Cc: Yang Z Zhang , Keir Fraser , "xen-devel@lists.xensource.com" , Allen M Kay List-Id: xen-devel@lists.xenproject.org >>> On 14.12.10 at 09:46, Weidong Han wrote: > Jan Beulich wrote: >> And why is it that Xen panics when the BIOS pre-enabled x2apic >> mode without also enabling interrupt re-mapping, while Linux >> (afaict from looking at 2.6.32 code and output from an affected >> machine) simply enables interrupt re-mapping in this case? >> >> Thanks, Jan >> >> =20 > x2apic spec 2.9 section states: The default will be for the BIOS to = pass=20 > the control to the OS with the local x2APICs in xAPIC mode if all = x2APIC=20 > IDs reported by CPUID.0BH:EDX are less than 255, and in x2APIC mode = if=20 > there are any logical processor reporting its x2APIC ID at 255 or = greater. >=20 > If BIOS transitions to Xen with x2APIC enabled, there must be APIC ID = >=20 > 255, which requires interrupt remapping. So we added code to check = such=20 > BIOS issue. So what would you tell a customer of yours reporting native Linux to boot fine on a certain system, but Xen dying early? Also, from a purely technical perspective, it doesn't seem like interrupt re-mapping is a requirement for the BIOS: With native Linux booting fine, there obviously is no issue. And clearly, as long as the boot CPU has an APIC id fitting into 8 bits, interrupts ought to work fine without re-mapping. When the boot CPU has a wider APIC ID, obviously the BIOS won't be able to bring up the system (and hence the question of booting whatever OS on top of it would be mute anyway). Jan