From df06ce560ddfefde98bef822ec2020382059921f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <eperezma@redhat.com>
Date: Thu, 21 Jul 2022 15:38:55 +0200
Subject: [PATCH 10/32] vdpa: Avoid compiler to squash reads to used idx
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RH-Author: Eugenio Pérez <eperezma@redhat.com>
RH-MergeRequest: 108: Net Control Virtqueue shadow Support
RH-Commit: [10/27] b28789302d4f64749da26f413763f918161d9b70 (eperezmartin/qemu-kvm)
RH-Bugzilla: 1939363
RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
RH-Acked-by: Cindy Lu <lulu@redhat.com>
RH-Acked-by: Laurent Vivier <lvivier@redhat.com>

Bugzilla: https://bugzilla.redhat.com/1939363

Upstream Status: git://git.qemu.org/qemu.git

commit c381abc37f0aba42ed2e3b41cdace8f8438829e4
Author: Eugenio Pérez <eperezma@redhat.com>
Date:   Wed Jul 20 08:59:29 2022 +0200

    vdpa: Avoid compiler to squash reads to used idx

    In the next patch we will allow busypolling of this value. The compiler
    have a running path where shadow_used_idx, last_used_idx, and vring used
    idx are not modified within the same thread busypolling.

    This was not an issue before since we always cleared device event
    notifier before checking it, and that could act as memory barrier.
    However, the busypoll needs something similar to kernel READ_ONCE.

    Let's add it here, sepparated from the polling.

    Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
    Signed-off-by: Jason Wang <jasowang@redhat.com>

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 3fbda1e3d4..9c46c3a8fa 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -327,11 +327,12 @@ static void vhost_handle_guest_kick_notifier(EventNotifier *n)
 
 static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
 {
+    uint16_t *used_idx = &svq->vring.used->idx;
     if (svq->last_used_idx != svq->shadow_used_idx) {
         return true;
     }
 
-    svq->shadow_used_idx = cpu_to_le16(svq->vring.used->idx);
+    svq->shadow_used_idx = cpu_to_le16(*(volatile uint16_t *)used_idx);
 
     return svq->last_used_idx != svq->shadow_used_idx;
 }
-- 
2.31.1