Re: [PATCH RFC] Drivers: hv: balloon: Temporary disable the driver on ARM64 when PAGE_SIZE != 4k

[Wei Liu <wei.liu@kernel.org>]

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@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
>