From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH LINUX RFC] xen: privcmd: implement IOCTL_PRIVCMD_MMAPBATCH_V2_UNCACHED Date: Fri, 20 Dec 2013 09:38:04 -0500 Message-ID: <20131220143804.GA29778@phenom.dumpdata.com> References: <1387387710.28680.88.camel@kazak.uk.xensource.com> <1387387837-9922-1-git-send-email-ian.campbell@citrix.com> <20131218211630.GB11717@phenom.dumpdata.com> <1387531153.1025.66.camel@dagon.hellion.org.uk> <20131220141332.GA28996@phenom.dumpdata.com> <1387549128.17289.87.camel@kazak.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1Vu1EP-0006S4-Ex for xen-devel@lists.xenproject.org; Fri, 20 Dec 2013 14:39:13 +0000 Content-Disposition: inline In-Reply-To: <1387549128.17289.87.camel@kazak.uk.xensource.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: Ian Campbell Cc: stefano.stabellini@eu.citrix.com, julien.grall@linaro.org, tim@xen.org, xen-devel@lists.xen.org, David Vrabel , xen-devel@lists.xenproject.org, Boris Ostrovsky List-Id: xen-devel@lists.xenproject.org On Fri, Dec 20, 2013 at 02:18:48PM +0000, Ian Campbell wrote: > On Fri, 2013-12-20 at 09:13 -0500, Konrad Rzeszutek Wilk wrote: > > On Fri, Dec 20, 2013 at 09:19:13AM +0000, Ian Campbell wrote: > > > On Wed, 2013-12-18 at 16:16 -0500, Konrad Rzeszutek Wilk wrote: > > > > On Wed, Dec 18, 2013 at 05:30:37PM +0000, Ian Campbell wrote: > > > > > On ARM we want to use uncached foreign mappings while building the domain > > > > > because the guests start with MMU and caches disabled. > > > > > > > > > > > > > Why introduce a new ioctl? Could we piggyback on the old one and on ARM > > > > do the uncached. > > > > > > Because there are (going to be) other circumstances where we do want a > > > cached foreign mapping, specifically migration. > > > > Why not then just expand the existing ioctl with a flag? ARM after all > > is still experimental so you could do that. > > Well, ARM just uses the same ioctl as on x86, so we wouldn't be > expanding, but duplicating, which didn't seem worth it. > > And secondly, unlike hypercalls, the ioctls are part of Linux's > interface, so I'm not sure how changing them would go down. You wouldn't break - expanding the structure still preserves the ABI. Only more recent toolstacks would know how to use it. But I get your point - you are hesistant to modify it the existing one. How about making this new more flexible - perhaps have a 'flag' and 'version' or even 'size' to account for changes? And call it IOCTL_.._V3 which would allows us to add more things in the future (depending on the version size)? That way if something else likes this pops up we have a nice escape vehicle ready. > > Ian. >