On 09/12/16 10:59, Jan Beulich wrote: >>>> On 08.12.16 at 18:22, <andrew.coop...@citrix.com> 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. > > Right; a first question though would be whether 5-level support > would be a build time selection (just like 32-bit PAE was long ago), > or runtime determined.
Guessing you mean Linux kernel here: the intention is to have one kernel being capable to use 4- or 5-level paging in the end. Current patches don't support this, they are using a Kconfig option to select the number of page table levels. >> 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. > > I agree, but we may face resistance from (Linux and other) kernel > folks. > >> However, a big question is whether any of this effort is worth doing, in >> the light of PVH. > > Much depends, I think, on how quickly this becomes a fully > supported feature. "this" refers to PVH, I guess? Juergen _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
