[PATCH] KVM: PPC: Book3S HV: XIVE: Free previous EQ page when setting up a new one

[Greg Kurz]

The EQ page is allocated by the guest and then passed to the hypervisor
with the H_INT_SET_QUEUE_CONFIG hcall. A reference is taken on the page
before handing it over to the HW. This reference is dropped either when
the guest issues the H_INT_RESET hcall or when the KVM device is released.
But, the guest can legitimately call H_INT_SET_QUEUE_CONFIG several times
to reset the EQ (vCPU hot unplug) or set a new EQ (guest reboot). In both
cases the EQ page reference is leaked. This is especially visible when
the guest memory is backed with huge pages: start a VM up to the guest
userspace, either reboot it or unplug a vCPU, quit QEMU. The leak is
observed by comparing the value of HugePages_Free in /proc/meminfo before
and after the VM is run.

Note that the EQ reset path seems to be calling put_page() but this is
done after xive_native_configure_queue() which clears the qpage field
in the XIVE queue structure, ie. the put_page() block is a nop and the
previous page pointer was just overwritten anyway. In the other case of
configuring a new EQ page, nothing seems to be done to release the old
one.

Fix both cases by always calling put_page() on the existing EQ page in
kvmppc_xive_native_set_queue_config(). This is a seemless change for the
EQ reset case. However this causes xive_native_configure_queue() to be
called twice for the new EQ page case: one time to reset the EQ and another
time to configure the new page. This is needed because we cannot release
the EQ page before calling xive_native_configure_queue() since it may still
be used by the HW. We cannot modify xive_native_configure_queue() to drop
the reference either because this function is also used by the XICS-on-XIVE
device which requires free_pages() instead of put_page(). This isn't a big
deal anyway since H_INT_SET_QUEUE_CONFIG isn't a hot path.

Reported-by: Satheesh Rajendran <sathnaga at linux.vnet.ibm.com>
Cc: stable at vger.kernel.org # v5.2
Fixes: 13ce3297c576 ("KVM: PPC: Book3S HV: XIVE: Add controls for the EQ configuration")
Signed-off-by: Greg Kurz <groug at kaod.org>
---
 arch/powerpc/kvm/book3s_xive_native.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_xive_native.c b/arch/powerpc/kvm/book3s_xive_native.c
index 34bd123fa024..8ab908d23dc2 100644
--- a/arch/powerpc/kvm/book3s_xive_native.c
+++ b/arch/powerpc/kvm/book3s_xive_native.c
@@ -570,10 +570,12 @@ static int kvmppc_xive_native_set_queue_config(struct kvmppc_xive *xive,
 		 __func__, server, priority, kvm_eq.flags,
 		 kvm_eq.qshift, kvm_eq.qaddr, kvm_eq.qtoggle, kvm_eq.qindex);
 
-	/* reset queue and disable queueing */
-	if (!kvm_eq.qshift) {
-		q->guest_qaddr  = 0;
-		q->guest_qshift = 0;
+	/*
+	 * Reset queue and disable queueing. It will be re-enabled
+	 * later on if the guest is configuring a new EQ page.
+	 */
+	if (q->guest_qshift) {
+		page = virt_to_page(q->qpage);
 
 		rc = xive_native_configure_queue(xc->vp_id, q, priority,
 						 NULL, 0, true);
@@ -583,12 +585,13 @@ static int kvmppc_xive_native_set_queue_config(struct kvmppc_xive *xive,
 			return rc;
 		}
 
-		if (q->qpage) {
-			put_page(virt_to_page(q->qpage));
-			q->qpage = NULL;
-		}
+		put_page(page);
 
-		return 0;
+		if (!kvm_eq.qshift) {
+			q->guest_qaddr  = 0;
+			q->guest_qshift = 0;
+			return 0;
+		}
 	}
 
 	/*