From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arjan van de Ven Subject: Re: Should KVM_GUEST stop depending on PARAVIRT? Date: Mon, 27 Jul 2015 12:04:55 -0700 Message-ID: <55B680D7.1000206@linux.intel.com> References: <55B63933.4090702@redhat.com> <55B66F42.3000202@redhat.com> <55B678B5.8040602@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: KVM list To: Andy Lutomirski , Paolo Bonzini Return-path: Received: from mga02.intel.com ([134.134.136.20]:15808 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752790AbbG0TE7 (ORCPT ); Mon, 27 Jul 2015 15:04:59 -0400 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 7/27/2015 11:45 AM, Andy Lutomirski wrote: >> With PARAVIRT=y it never #GPs: >> >> .read_msr = native_read_msr_safe, >> .write_msr = native_write_msr_safe, >> >> I don't remember if it's this way on bare-metal too. > > Oh, whoops, I missed the "_safe". IMO that's just a bug, and I guess > KVM relies on it? btw it's a common misperception that _safe is actually safe;-) they're still highly dangerous, just they won't fault on the linux side > > ISTM the host should be fixed so that a non-PARAVIRT guest won't crash > when using perf (if it indeed currently crashes) and/or the perf code > should be fixed to work without this bug^Wfeature. > > Then KVM_GUEST kernels could be de-bloated by dropping PARAVIRT. > > Hi Arjan- A quick and dirty measurement suggests that this would save > 2-3 ms when booting a KVM_GUEST=y kernel under KVM by turning > apply_paravirt into a noop. nice