On 24.11.20 19:11, Jonathan Cameron wrote: > On Mon, 9 Nov 2020 20:47:09 +0100 > David Hildenbrand <da...@redhat.com> wrote: > > +CC Eric based on similar query in other branch of the thread. > >> On 05.11.20 18:43, Jonathan Cameron wrote: >>> Basically a cut and paste job from the x86 support with the exception of >>> needing a larger block size as the Memory Block Size (MIN_SECTION_SIZE) >>> on ARM64 in Linux is 1G. >>> >>> Tested: >>> * In full emulation and with KVM on an arm64 server. >>> * cold and hotplug for the virtio-mem-pci device. >>> * Wide range of memory sizes, added at creation and later. >>> * Fairly basic memory usage of memory added. Seems to function as normal. >>> * NUMA setup with virtio-mem-pci devices on each node. >>> * Simple migration test. >>> >>> Related kernel patch just enables the Kconfig item for ARM64 as an >>> alternative to x86 in drivers/virtio/Kconfig >>> >>> The original patches from David Hildenbrand stated that he thought it should >>> work for ARM64 but it wasn't enabled in the kernel [1] >>> It appears he was correct and everything 'just works'. >>> >>> The build system related stuff is intended to ensure virtio-mem support is >>> not built for arm32 (build will fail due no defined block size). >>> If there is a more elegant way to do this, please point me in the right >>> direction. >> >> You might be aware of https://virtio-mem.gitlab.io/developer-guide.html >> and the "issue" with 64k base pages - 512MB granularity. Similar as the >> question from Auger, have you tried running arm64 with differing page >> sizes in host/guest? >> > > Hi David, > >> With recent kernels, you can use "memhp_default_state=online_movable" on >> the kernel cmdline to make memory unplug more likely to succeed - >> especially with 64k base pages. You just have to be sure to not hotplug >> "too much memory" to a VM. > > Thanks for the pointer - that definitely simplifies testing. Was getting a > bit > tedious with out that. > > As ever other stuff got in the way, so I only just got back to looking at > this. > > I've not done a particularly comprehensive set of tests yet, but things seem > to 'work' with mixed page sizes. > > With 64K pages in general, you run into a problem with the device block_size > being > smaller than the subblock_size. I've just added a check for that into the
"device block size smaller than subblock size" - that's very common, e.g., on x86-64. E.g., device_block_size is 2MiB, subblock size 4MiB - until we improve that in the future in Linux guests. Or did you mean something else? > virtio-mem kernel driver and have it fail to probe if that happens. I don't > think such a setup makes any sense anyway so no loss there. Should it make > sense > to drop that restriction in the future we can deal with that then without > breaking > backwards compatibility. > > So the question is whether it makes sense to bother with virtio-mem support > at all on ARM64 with 64k pages given currently the minimum workable block_size > is 512MiB? I guess there is an argument of virtio-mem being a possibly more > convenient interface than full memory HP. Curious to hear what people think > on > this? IMHO we really want it. For example, RHEL is always 64k. This is a current guest limitation, to be improved in the future - either by moving away from 512MB huge pages with 64k or by improving alloc_contig_range(). > > 4K guest on 64K host seems fine and no such limit is needed - though there > may be performance issues doing that. Yeah, if one is lucky to get one of these 512 MiB huge pages at all :) > > 64k guest on 4k host with 512MiB block size seems fine. > > If there are any places anyone thinks need particular poking I'd appreciate a > hint :) If things seem to work for now, that's great :) Thanks! -- Thanks, David / dhildenb
