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SubjectRe: [PATCH RFC] Drivers: hv: balloon: Temporary disable the driver on ARM64 when PAGE_SIZE != 4k
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Wei Liu <wei.liu@kernel.org> 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@redhat.com>
>> ---
>> 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

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