From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [Patch v5 1/4] Remove SMEP bit from CR4_RESERVED_BITS Date: Mon, 30 May 2011 10:05:52 +0200 Message-ID: <20110530080552.GG27557@elte.hu> References: <5D8008F58939784290FAB48F5497519844E92781DF@shsmsx502.ccr.corp.intel.com> <20110530074033.GB27557@elte.hu> <4DE34BF9.20106@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Yang, Wei Y" , Pekka Enberg , "kvm@vger.kernel.org" To: Avi Kivity Return-path: Received: from mx2.mail.elte.hu ([157.181.151.9]:43154 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752538Ab1E3IGG (ORCPT ); Mon, 30 May 2011 04:06:06 -0400 Content-Disposition: inline In-Reply-To: <4DE34BF9.20106@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: * Avi Kivity wrote: > On 05/30/2011 10:40 AM, Ingo Molnar wrote: > >* Yang, Wei Y wrote: > > > >> This patch removes SMEP bit from CR4_RESERVED_BITS. > > > >I'm wondering, what is the best-practice way for tools/kvm/ to set > >SMEP for the guest kernel automatically, even if the guest kernel > >itsef has not requested SMEP? > > > > The portion i'm worried about are old KVM versions that have the > > SMEP bit in CR4_RESERVED_BITS and reject it. So we cannot just > > unilaterally add SMEP to every cr4 write of the guest. > > tools/kvm doesn't see cr4 writes at all. [...] I feared small complications like that! :-) We can definitely use KVM_GET_SREGS, fiddle the SMEP bit in kvm_regs.cr4 and call KVM_SET_SREGS, once the fine patch above goes upstream. > [...] The only way to do this is in kvm itself. > > > Is there a way to query whether the host KVM version supports > > SMEP setting in cr4? > > > > KVM_GET_SUPPORTED_CPUID (it returns whether both the host cpu and > kvm support smep; if one of them doesn't, you'll see smep > disabled). That looks useful. So the way to go appears to be to do a GET_SREGS/SET_SREGS sequence to enable SMEP in the guest, some time after it has booted and has enabled paging. I'm wondering whether there's a suitable place to do that, when we are more or less guaranteed to exit the VM for some other reason - such as the first MMIO done with paging enabled? This solution means that we'll slow down pre-paging MMIOs with a GET_SREGS call, but that's ok, they are rare and the pre-paging bootup phase is very short. So the only worry would be where the guest sets cr4 itself - and since it does not know about SMEP it will probably disable it. Guest suspend/resume is one such place ... Another option would be to try to set the SMEP bit *before* we enable paging. In theory this should not confuse a Linux guest - and while i have not tested it i *think* we let it survive in the saved_cr4_features shadow variable. That would make guest suspend/resume work out of box as well. Thanks, Ingo