From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 4/5] export new cpuid KVM_CAP Date: Sat, 17 Apr 2010 21:58:26 +0300 Message-ID: <4BCA04D2.1040300@redhat.com> References: <1271356648-5108-1-git-send-email-glommer@redhat.com> <1271356648-5108-2-git-send-email-glommer@redhat.com> <1271356648-5108-3-git-send-email-glommer@redhat.com> <1271356648-5108-4-git-send-email-glommer@redhat.com> <1271356648-5108-5-git-send-email-glommer@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Glauber Costa Return-path: In-Reply-To: <1271356648-5108-5-git-send-email-glommer@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 04/15/2010 09:37 PM, Glauber Costa wrote: > Since we're changing the msrs kvmclock uses, we have to communicate > that to the guest, through cpuid. We can add a new KVM_CAP to the > hypervisor, and then patch userspace to recognize it. > > And if we ever add a new cpuid bit in the future, we have to do that again, > which create some complexity and delay in feature adoption. > > Instead, what I'm proposing in this patch is a new capability, called > KVM_CAP_X86_CPUID_FEATURE_LIST, that returns the current feature list > currently supported by the hypervisor. If we ever want to add or remove > some feature, we only need to tweak into the HV, leaving userspace untouched. > > Hm. We need to update userspace anyway, since we don't like turning features on unconditionally (it breaks live migration into an older kernel). -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.