From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: Xen Project policy on feature flags Date: Mon, 29 Sep 2014 11:05:29 +0100 Message-ID: <54292EE9.9090708@citrix.com> References: <542588A90200007800039B3E@mail.emea.novell.com> <1411741172.26149.75.camel@kazak.uk.xensource.com> <20140926142959.GA19421@laptop.dumpdata.com> <54291F9D.4030705@eu.citrix.com> <20140929093126.GF28628@zion.uk.xensource.com> <54292823.2050502@eu.citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54292823.2050502@eu.citrix.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: George Dunlap , Wei Liu Cc: xen-devel@lists.xensource.com, keir@xen.org, Tim Deegan , Stefano Stabellini , Ian Jackson , Lars Kurth , Jan Beulich , Ian Campbell List-Id: xen-devel@lists.xenproject.org On 29/09/14 10:36, George Dunlap wrote: > On 09/29/2014 10:31 AM, Wei Liu wrote: >> On Mon, Sep 29, 2014 at 10:00:13AM +0100, George Dunlap wrote: >>> On 09/26/2014 03:49 PM, Stefano Stabellini wrote: >>>>> Let me rephrase - will it boot in the same fashion (And with the same >>>>> bugs) as it did prior to this functionality being introduced? >>>> 3.15 -> dom0 on ARM broken (if netback is used) >>>> 3.17 -> dom0 on ARM is fixed, only if the kernel is compiled with >>>> CONFIG_ARM_LPAE >>>> >>>> Reverting the XENFEAT_grant_map_identity related changes would give you >>>> a system broken even with CONFIG_ARM_LPAE. >>>> Reverting Zoltan's changes to netback would give you a working system. >> FWIW reverting isn't practical as many more fixes have gone in. >> >> I think a possible workaround is to copy directly xen-netback directory >> from 3.14 and build it against new kernel. Netback itself is quite >> self-contained. > > Could we provide a patch which would just disable the problematic behavior? No. This would require re-introducing the grant copy from-guest path to netback. This would be expensive since netback has seen significant changes since (multi-queue support in particular). It would also not fix the underlying ARM-specific bug and other users of grant mapping would be similarly broken. I think we should: 1. Revert XENFEAT_grant_map_identity. 2. Add the flush-cache-by-bus-address hypercall. 3. Add the Linux support this this cache operation and tag this for stable. 4. Backport the hypercall to 4.4.x. I think this is critical to fix in 4.5 and should have a freeze exception. I would even consider slipping the 4.5 release to get this fixed. David