[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Future support of 5-level paging in Xen:wq
On Thu, 8 Dec 2016, Andrew Cooper wrote: > On 08/12/2016 19:18, Stefano Stabellini wrote: > > On Thu, 8 Dec 2016, Andrew Cooper wrote: > >> On 08/12/16 16:46, Juergen Gross wrote: > >>> The first round of (very preliminary) patches for supporting the new > >>> 5-level paging of future Intel x86 processors [1] has been posted to > >>> lkml: > >>> > >>> https://lkml.org/lkml/2016/12/8/378 > >>> > >>> An explicit note has been added: "CONFIG_XEN is broken." and > >>> "I would appreciate help with the code." > >>> > >>> I think we should start a discussion what we want to do in future: > >>> > >>> - are we going to support 5-level paging for PV guests? > >>> - do we limit 5-level paging to PVH and HVM? > >> The 64bit PV ABI has 16TB of virtual address space just above the upper > >> 48-canonical boundary. > >> > >> Were Xen to support 5-level PV guests, we'd either leave the PV guest > >> kernel with exactly the same amount of higher half space as it currently > >> has, or we'd have to recompile Xen as properly position-independent and > >> use a different virtual range in different paging mode. > >> > >> Another pain point is the quantity of virtual address space handed away > >> in the ABI. We currently had 97% of the virtual address space away to > >> 64bit PV guests, and frankly this is too much. This is the wrong way > >> around when Xen has more management to do than the guest. If we were to > >> go along the 5-level PV guests route, I will insist that there is a > >> rather more even split of virtual address space baked into the ABI. > >> > >> However, a big question is whether any of this effort is worth doing, in > >> the light of PVH. > > With my Aporeto hat on, I'll say that given the overwhelming amount of > > hardware available out there without virtualization support, this work > > is worth doing. I am referring to all the public cloud virtual machines, > > which can support Xen PV guests but cannot support PVH guests. > > Why is Xen supporting 5-level guests useful for running in a PV cloud > VM? Xen doesn't run PV. > > I am not suggesting that we avoid adding 5-level support to Xen. We > should absolutely do that. The question is only whether we extend the > PV ABI to support 5-level PV guests. Conceptually, its very easy to > have a 5-level Xen but only supporting 4-level PV guests. > > VT-x and SVM date from 2005/2006 and are now 10 years old. I would be > surprised if you would find much hardware of this age in any cloud; you > can't by anything that old, and support contracts have probably run out > if you have owned that hardware for 10 years. I am thinking that in a couple of years, we might already find VMs so large that to use all the memory in a nested virt scenario, we need 5-level PV abi support. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
|
![]() |
Lists.xenproject.org is hosted with RackSpace, monitoring our |