Wei Liu <wei.liu@xxxxxxxxxx> writes: > 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? > Let's ask the expert) David, how does virtio-balloon (and virtio-mem) deal with different page sizes between guest and host? > >> 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 >> > -- Vitaly
