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From:   "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To:     Boqun Feng <boqun.feng@gmail.com>, Wei Liu <wei.liu@kernel.org>
CC:     vkuznets <vkuznets@redhat.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        David Hildenbrand <david@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC v1.1] Drivers: hv: balloon: Disable balloon and hot-add
 accordingly
Thread-Topic: [RFC v1.1] Drivers: hv: balloon: Disable balloon and hot-add
 accordingly
Thread-Index: AQHYKe3uniebtwYsQEWUHIDMFsz9UqykfosA
Date:   Fri, 25 Feb 2022 17:06:45 +0000
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References: <20220223131548.2234326-3-boqun.feng@gmail.com>
 <20220225021714.3815691-1-boqun.feng@gmail.com>
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From: Boqun Feng <boqun.feng@gmail.com> Sent: Thursday, February 24, 2022 6=
:17 PM
>=20
> Currently there are known potential issues for balloon and hot-add on
> ARM64:
>=20
> *	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.

>=20
> *	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().
>=20
> 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.

>=20
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> v1 --> v1.1:
>=20
> *	Use HV_HYP_PAGE_SIZE instead of hard coding 4096 as suggested by
> 	Michael.
>=20
> *	Explicitly print out the disable message if a function is
> 	disabled as suggested by Michael.
>=20
>  drivers/hv/hv_balloon.c | 36 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)
>=20
> 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)
>  	}
>  }
>=20
> +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 clear=
ly
don't guarantee page ranges are guest page aligned.

> +	 */
> +	if (PAGE_SIZE !=3D 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 *de=
v)
>  	 * 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 =3D 1;
> -	cap_msg.caps.cap_bits.hot_add =3D 1;
> +	cap_msg.caps.cap_bits.balloon =3D ballooning_enabled();
> +	cap_msg.caps.cap_bits.hot_add =3D hot_add_enabled();
>=20
>  	/*
>  	 * Specify our alignment requirements as it relates
> --
> 2.33.0

The code looks good to me.

Michael