From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [PATCH V3 1/1] expand x86 arch_shared_info to support >3 level p2m tree Date: Tue, 16 Sep 2014 11:14:48 +0100 Message-ID: <54180D98.8030903@citrix.com> References: <1410256709-25885-1-git-send-email-jgross@suse.com> <1410256709-25885-2-git-send-email-jgross@suse.com> <540ED600.3060102@citrix.com> <540EDB4F.30402@suse.com> <5412CB80.9030208@suse.com> <5416A379.5@citrix.com> <5416A8CE.5020400@suse.com> <5416B518.8030504@citrix.com> <5416B6F1.2020102@suse.com> <5416BFC2.4040900@citrix.com> <5416C392.1010707@suse.com> <5416F809.7060509@citrix.com> <5417B40B.4000703@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5417B40B.4000703@suse.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: Juergen Gross , David Vrabel , Andrew Cooper , ian.campbell@citrix.com, ian.jackson@eu.citrix.com, jbeulich@suse.com, keir@xen.org, tim@xen.org, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On 16/09/14 04:52, Juergen Gross wrote: > On 09/15/2014 04:30 PM, David Vrabel wrote: >> On 15/09/14 11:46, Juergen Gross wrote: >>> So you'd prefer: >>> >>> 1) >512GB pv-domains (including Dom0) will be supported only with new >>> Xen (4.6?), no matter if the user requires migration to be supported >> >> Yes. >512 GiB and not being able to migrate are not obviously related >> from the point of view of the end user (unlike assigning a PCI device). >> >> Failing at domain save time is most likely too late for the end user. > > What would you think about following compromise: > > We add a flag that indicates support of multi-level p2m. Additionally > the Linux kernel can ignore the flag not being set either if started as > Dom0 or if told so via kernel parameter. This sounds fine but this override should be via the command line parameter only. Crash dump analysis tools may not understand the 4 level p2m. >>> to: >>> >>> 2) >512GB pv-domains (especially Dom0 and VMs with direct hw access) can >>> be started on current Xen versions, migration is possible only if >>> Xen >>> is new (4.6?) >> >> There's also my preferred option: >> >> 3) >512 GiB PV domains are not supported. Large guests must be PVH or >> PVHVM. > > In theory okay, but not right now, I think. PVH Dom0 is not production > ready. I'm not really seeing the need for such a large dom0. I remain unconvinced that there are sufficient use cases to justify extending the PV only ABI and increasing complexity of the current 3-level p2m code. I'm concerned that 4-level p2m support will impact the performance of guests that do not need the 4 levels. It may be necessary to use the alternatives mechanism to select the correct low-level lookup function. I also think a flat array for the p2m might be better (less complex). There's plenty of virtual address space in a 64-bit guest to allow for this. David