Re: [PATCH v2] hw/arm/virt enable support for virtio-mem

[Andrew Jones]

On Wed, Nov 25, 2020 at 09:45:19AM +0100, David Hildenbrand wrote:
> On 25.11.20 09:38, Andrew Jones wrote:
> > On Tue, Nov 24, 2020 at 08:17:35PM +0100, David Hildenbrand wrote:
> >> On 24.11.20 19:11, Jonathan Cameron wrote:
> >>> On Mon, 9 Nov 2020 20:47:09 +0100
> >>> David Hildenbrand <david@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().
> > 
> > Even with 64k pages you may be able to have 2MB huge pages by setting
> > default_hugepagesz=2M on the kernel command line.
> 
> Yes, but not for THP, right? Last time I checked that move was not
> performed yet - resulting in MAX_ORDER/pageblock_order in Linux
> corresponding to 512 MB.
>

Yes, I believe you're correct. At least on the machine I've booted with
default_hugepagesz=2M, I see

 $ cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
 536870912

(I'm not running a latest mainline kernel though.)

Thanks,
drew