From: Zachary Amsden Subject: Re: [RFC PATCH 18/33] Subarch support for CPUID instruction Date: Tue, 18 Jul 2006 04:33:52 -0700 Message-ID: <44BCC720.7050601@vmware.com> References: <20060718091807.467468000@sous-sol.org> <20060718091953.003336000@sous-sol.org> <1153217686.3038.37.camel@laptopd505.fenrus.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1153217686.3038.37.camel@laptopd505.fenrus.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Arjan van de Ven Cc: Andrew Morton , Jeremy Fitzhardinge , xen-devel@lists.xensource.com, Ian Pratt , Rusty Russell , linux-kernel@vger.kernel.org, Andi Kleen , Chris Wright , virtualization@lists.osdl.org, Christian Limpach List-Id: xen-devel@lists.xenproject.org Arjan van de Ven wrote: > On Tue, 2006-07-18 at 00:00 -0700, Chris Wright wrote: > >> plain text document attachment (i386-cpuid) >> Allow subarchitectures to modify the CPUID instruction. This allows >> the subarch to provide a limited set of CPUID feature flags during CPU >> identification. Add a subarch implementation for Xen that traps to the >> hypervisor where unsupported feature flags can be hidden from guests. >> > > Hi, > > I'm wondering if this is entirely the wrong level of abstraction; to me > it feels the subarch shouldn't override the actual cpuid, but the cpu > feature flags that linux uses. That's a lot less messy: cpuid has many > many pieces of information which are near impossible to filter in > practice, however filtering the USAGE of it is trivial; linux basically > flattens the cpuid namespace into a simple bitmap of "what the kernel > can use". That is really what the subarch should filter/fixup, just like > we do for cpu quirks etc etc. > You really need a CPUID hook. The instruction is non-virtualizable, and anything claiming to be a hypervisor really has to support masking and flattening the cpuid namespace for the instruction itself. It is used in assembler code and very early in boot. The alternative is injecting a bunch of Xen-specific code to filter feature bits into the i386 layer, which is both bad for Linux and bad for Xen - and was quite ugly in the last set of Xen patches. Zach