On Wed, Jan 05, 2022 at 05:50:28PM +0100, Vitaly Kuznetsov wrote: > Hyper-V ballooning and memory hotplug protocol always seems to assume > 4k page size so all PFNs in the structures used for communication are > 4k PFNs. In case a different page size is in use on the guest (e.g. > 64k), things go terribly wrong all over: > - When reporting statistics, post_status() reports them in guest pages > and hypervisor sees very low memory usage. > - When ballooning, guest reports back PFNs of the allocated pages but > the hypervisor treats them as 4k PFNs. > - When unballooning or memory hotplugging, PFNs coming from the host > are 4k PFNs and they may not even be 64k aligned making it difficult > to handle. > > While statistics and ballooning requests would be relatively easy to > handle by converting between guest and hypervisor page sizes in the > communication structures, handling unballooning and memory hotplug > requests seem to be harder. In particular, when ballooning up > alloc_balloon_pages() shatters huge pages so unballooning request can > be handled for any part of it. It is not possible to shatter a 64k > page into 4k pages so it's unclear how to handle unballooning for a > sub-range if such request ever comes so we can't just report a 64k > page as 16 separate 4k pages. > How does virtio-balloon handle it? Does its protocol handle different page sizes? > Ideally, the protocol between the guest and the host should be changed > to allow for different guest page sizes. > > While there's no solution for the above mentioned problems, it seems > we're better off without the driver in problematic cases. > > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> > --- > drivers/hv/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig > index 0747a8f1fcee..fb353a13e5c4 100644 > --- a/drivers/hv/Kconfig > +++ b/drivers/hv/Kconfig > @@ -25,7 +25,7 @@ config HYPERV_UTILS > > config HYPERV_BALLOON > tristate "Microsoft Hyper-V Balloon driver" > - depends on HYPERV > + depends on HYPERV && (X86 || (ARM64 && ARM64_4K_PAGES)) > select PAGE_REPORTING > help > Select this option to enable Hyper-V Balloon driver. > -- > 2.33.1 >
