From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: [PATCH] x86: add feature flags to shared_info page Date: Mon, 02 Mar 2015 16:11:44 +0100 Message-ID: <54F47DB0.9020007@suse.com> References: <1425301144-27119-1-git-send-email-jgross@suse.com> <54F470740200007800065076@mail.emea.novell.com> <54F4694D.3010808@citrix.com> <54F47A2C0200007800065109@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1YSS0f-0004YB-Vs for xen-devel@lists.xenproject.org; Mon, 02 Mar 2015 15:11:54 +0000 In-Reply-To: <54F47A2C0200007800065109@mail.emea.novell.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: Jan Beulich , Andrew Cooper Cc: keir@xen.org, Ian.Campbell@citrix.com, tim@xen.org, ian.jackson@eu.citrix.com, david.vrabel@citrix.com, xen-devel@lists.xenproject.org List-Id: xen-devel@lists.xenproject.org On 03/02/2015 02:56 PM, Jan Beulich wrote: >>>> On 02.03.15 at 14:44, wrote: >> On 02/03/15 13:15, Jan Beulich wrote: >>>>>> On 02.03.15 at 13:59, <"jgross@suse.com".non-mime.internet> wrote: >>>> In order to indicate the Xen tools capability to support the virtual >>>> mapped linear p2m list instead the 3 level mfn tree add feature flags >>>> to the shared_info page. >>> But why in the shared info page? They'd belong to start info or should >>> be obtainable via XENVER_get_features. >> >> Furthermore in this case, the virtual linear p2m is purely a >> guest->toolstack feature/ABI. >> >> Xen deliberately has no knowledge of PV guest p2ms of either the 3level >> or linear variety. >> >> Is there genuinely no better interface than the hypervisor feature flags >> to indicate a piece of toolstack support? > > As Ian indicated in his reply, much depends on whether any other > mechanism would allow the information to be retrieved early enough > in the guest. Options that would work are, regarding the time the information is needed: - XENVER_get_features - start info - shared info - any (other) hypercall All other interfaces are available much too late. I think start info is the best option, as it is built by the tools for domUs and, as Jan already mentioned, would be a better place for the information as shared info. The last remaining question: what to do regarding dom0? Here I see the following alternatives: - do nothing: the 3 level mfn tree is built even if it is not needed, wastes up to 2MB memory and might slow down dom0 (I doubt the slow down would be detectable) - set the flag based on a hypervisor boot parameter (not very nice) - throw the 3 level mfn tree away during boot as soon as the tools can tell dom0 to do so (requires a new interface) Any thoughts? Juergen