From mboxrd@z Thu Jan 1 00:00:00 1970 From: Borislav Petkov Subject: Re: [PATCH 1/6] kvm: Add KVM_GET_EMULATED_CPUID Date: Thu, 26 Sep 2013 20:55:24 +0200 Message-ID: <20130926185524.GA10123@pd.tnic> References: <1379861095-628-1-git-send-email-bp@alien8.de> <1379861095-628-2-git-send-email-bp@alien8.de> <20130923162856.GC7264@otherpad.lan.raisama.net> <2f5d83d4d90ba9c5930f099d6f73e61b.squirrel@www.skyhub.de> <20130924100414.GE17294@redhat.com> <20130926141915.GV2840@otherpad.lan.raisama.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: Gleb Natapov , LKML , Borislav Petkov , "H. Peter Anvin" , Paolo Bonzini , Andre Przywara , Joerg Roedel , X86 ML , KVM , qemu-devel@nongnu.org, libvir-list@redhat.com, Jiri Denemark To: Eduardo Habkost Return-path: Content-Disposition: inline In-Reply-To: <20130926141915.GV2840@otherpad.lan.raisama.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Thu, Sep 26, 2013 at 11:19:15AM -0300, Eduardo Habkost wrote: > Then we may have a problem: some CPU models already have "movbe" > included (e.g. Haswell), and patch 6/6 will make "-cpu Haswell" get > movbe enabled even if it is being emulated. Huh? HSW has MOVBE so we won't #UD on it and MOVBE will get executed in hardware when executing the guest. IOW, we'll never get to the emulation path of piggybacking on the #UD. > So if we really want to avoid enabling emulated features by mistake, > we may need a new CPU flag in addition to "enforce" to tell QEMU that > it is OK to enable emulated features (maybe "-cpu ...,emulate"?). EMULATED_CPUID are off by default and only if you request them specifically, they get enabled. If you start with "-cpu Haswell", MOVBE will be already set in the host CPUID. Or am I missing something? > But my question still stands: suppose we had x2apic emulation > implemented but for some reason it was painfully slow, we wouldn't > want to enable it by mistake. In this case, it would end up on > EMULATED_CPUID and not on SUPPORTED_CPUID, right? IMHO we want to enable emulation only when explicitly requested... regardless of the emulation performance. Thanks. -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. --