From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v2] x86/vm_event: toggle singlestep from vm_event response Date: Mon, 6 Jul 2015 18:44:13 +0100 Message-ID: <559ABE6D.6080303@citrix.com> References: <1435673482-10674-1-git-send-email-tlengyel@novetta.com> <5592A51E.1050200@citrix.com> <559ABA50020000780008CD3A@mail.emea.novell.com> <559AC0D3020000780008CDFD@mail.emea.novell.com> <559AB4E7.1040506@bitdefender.com> <559AB83B.4030907@citrix.com> <559ABB0F.2030300@bitdefender.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <559ABB0F.2030300@bitdefender.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Razvan Cojocaru , "Lengyel, Tamas" Cc: stefano.stabellini@citrix.com, keir@xen.org, Ian Campbell , Jan Beulich , Xen-devel List-Id: xen-devel@lists.xenproject.org On 06/07/15 18:29, Razvan Cojocaru wrote: > On 07/06/2015 08:17 PM, Andrew Cooper wrote: >> On 06/07/15 18:08, Lengyel, Tamas wrote: >>> Having said that (and with the understading that it is beyond the >>> scope >>> of this patch), a way to validate things like these is a good idea. I >>> wonder if, in a future patch, we could not have ./configure detect >>> these >>> things and simply disable the relevant VM_EVENT_FLAG constants with >>> #if(n)defs, for example. That way, you wouldn't be able to compile >>> code >>> that wouldn't work silently on platforms where that is the case. >>> >>> >>> It would be something worth investigating, definitely. >> It would be mad to conditionally compile out code based on the features >> or lackthereof of the build machine. >> >> For bits like this, there must be active negotiation between userspace >> and the running hypervisor to see what it can support. Imagine if the >> user disabled the monitor trap feature in the BIOS? Userspace cannot >> possibly assume that because it is running on Intel, that the feature is >> present and usable. > Fair enough, but it would at least compile out the code for machines > where it _definitely_ won't work, and on those where it _might_ work it > would just continue to do nothing silently (or perhaps with the > hypervisor logging an error) once the user disables the monitor trap > flag in the BIOS, so while not perfect it's still something, with the > benefit of less development overhead than a full-on system for > negotiating hypervisor capabilities (which is indeed the safest and > exhaustive course of action). That is fine if you are compiling a custom binary for your own use. It is not fine if you are a distro attempting to provide a binary for general use by a broad set of users. Code in tree should cater to this latter category. Compile-time feature detection like this is a step backwards from the 1993 days when mechanisms like `cpuid` were introduced. A one-time query of available features is nothing like as much overhead as having to recompile the binary. ~Andrew