linux-kernel - [RFC 2/2] Drivers: hv: balloon: Disable balloon and hot-add accordingly

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Date:   Wed, 23 Feb 2022 21:15:48 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        linux-hyperv@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>
Subject: [RFC 2/2] Drivers: hv: balloon: Disable balloon and hot-add accordingly

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.

*	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.
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.

Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 drivers/hv/hv_balloon.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 062156b88a87..35dcda20be85 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -1730,9 +1730,19 @@ static int balloon_connect_vsp(struct hv_device *dev)
 	 * When hibernation (i.e. virtual ACPI S4 state) is enabled, the host
 	 * 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.
+	 *
+	 * We disable balloon if the page size is larger than 4k, since
+	 * currently it's unclear to us whether an unballoon request can make
+	 * sure all page ranges are guest page size aligned.
+	 *
+	 * We also 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().
 	 */
-	cap_msg.caps.cap_bits.balloon = 1;
-	cap_msg.caps.cap_bits.hot_add = 1;
+	cap_msg.caps.cap_bits.balloon = !(PAGE_SIZE > 4096UL);
+	cap_msg.caps.cap_bits.hot_add = !IS_ENABLED(CONFIG_ARM64);
 
 	/*
 	 * Specify our alignment requirements as it relates
-- 
2.35.1