From: Boqun Feng <boqun.feng@xxxxxxxxx> Sent: Thursday, February 24, 2022 6:17 PM
>
> Currently there are known potential issues for balloon and hot-add on
> ARM64:
>
> * Unballoon requests from Hyper-V should only unballoon ranges
> that are guest page size aligned, otherwise guests cannot handle
> because it's impossible to partially free a page.
The above problem occurs only when the guest page size is > 4 Kbytes.
>
> * Memory hot-add requests from Hyper-V should provide the NUMA
> node id of the added ranges or ARM64 should have a functional
> memory_add_physaddr_to_nid(), otherwise the node id is missing
> for add_memory().
>
> These issues require discussions on design and implementation. In the
> meanwhile, post_status() is working and essiential to guest monitoring.
s/essiential/essential/
> Therefore instead of the entire hv_balloon driver, the balloon and
> hot-add are disabled accordingly for now. Once the issues are fixed,
> they can be re-enable in these cases.
Missing the word "disabling" in the first line? Also the balloon
function is disabled only if the page size is > 4 Kbytes.
>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
> v1 --> v1.1:
>
> * Use HV_HYP_PAGE_SIZE instead of hard coding 4096 as suggested by
> Michael.
>
> * Explicitly print out the disable message if a function is
> disabled as suggested by Michael.
>
> drivers/hv/hv_balloon.c | 36 ++++++++++++++++++++++++++++++++++--
> 1 file changed, 34 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 062156b88a87..eee7402cfc02 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -1660,6 +1660,38 @@ static void disable_page_reporting(void)
> }
> }
>
> +static int ballooning_enabled(void)
> +{
> + /*
> + * Disable ballooning if the page size is not 4k (HV_HYP_PAGE_SIZE),
> + * since currently it's unclear to us whether an unballoon request can
> + * make sure all page ranges are guest page size aligned.
My interpretation of the conversations with Hyper-V is that that they clearly
don't guarantee page ranges are guest page aligned.
> + */
> + if (PAGE_SIZE != HV_HYP_PAGE_SIZE) {
> + pr_info("Ballooning disabled because page size is not 4096 bytes\n");
> + return 0;
> + }
> +
> + return 1;
> +}
> +
> +static int hot_add_enabled(void)
> +{
> + /*
> + * Disable hot add on ARM64, because we currently rely on
> + * memory_add_physaddr_to_nid() to get a node id of a hot add range,
> + * however ARM64's memory_add_physaddr_to_nid() always return 0 and
> + * DM_MEM_HOT_ADD_REQUEST doesn't have the NUMA node information for
> + * add_memory().
> + */
> + if (IS_ENABLED(CONFIG_ARM64)) {
> + pr_info("Memory hot add disabled on ARM64\n");
> + return 0;
> + }
> +
> + return 1;
> +}
> +
> static int balloon_connect_vsp(struct hv_device *dev)
> {
> struct dm_version_request version_req;
> @@ -1731,8 +1763,8 @@ static int balloon_connect_vsp(struct hv_device *dev)
> * currently still requires the bits to be set, so we have to add code
> * to fail the host's hot-add and balloon up/down requests, if any.
> */
> - cap_msg.caps.cap_bits.balloon = 1;
> - cap_msg.caps.cap_bits.hot_add = 1;
> + cap_msg.caps.cap_bits.balloon = ballooning_enabled();
> + cap_msg.caps.cap_bits.hot_add = hot_add_enabled();
>
> /*
> * Specify our alignment requirements as it relates
> --
> 2.33.0
The code looks good to me.
Michael
