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From df06ce560ddfefde98bef822ec2020382059921f Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Eugenio=20P=C3=A9rez?= <eperezma@redhat.com>
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Date: Thu, 21 Jul 2022 15:38:55 +0200
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Subject: [PATCH 10/32] vdpa: Avoid compiler to squash reads to used idx
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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RH-Author: Eugenio Pérez <eperezma@redhat.com>
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RH-MergeRequest: 108: Net Control Virtqueue shadow Support
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RH-Commit: [10/27] b28789302d4f64749da26f413763f918161d9b70 (eperezmartin/qemu-kvm)
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RH-Bugzilla: 1939363
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RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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RH-Acked-by: Cindy Lu <lulu@redhat.com>
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RH-Acked-by: Laurent Vivier <lvivier@redhat.com>
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Bugzilla: https://bugzilla.redhat.com/1939363
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Upstream Status: git://git.qemu.org/qemu.git
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commit c381abc37f0aba42ed2e3b41cdace8f8438829e4
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Author: Eugenio Pérez <eperezma@redhat.com>
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Date: Wed Jul 20 08:59:29 2022 +0200
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vdpa: Avoid compiler to squash reads to used idx
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In the next patch we will allow busypolling of this value. The compiler
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have a running path where shadow_used_idx, last_used_idx, and vring used
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idx are not modified within the same thread busypolling.
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This was not an issue before since we always cleared device event
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notifier before checking it, and that could act as memory barrier.
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However, the busypoll needs something similar to kernel READ_ONCE.
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Let's add it here, sepparated from the polling.
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Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
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Signed-off-by: Jason Wang <jasowang@redhat.com>
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Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
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---
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hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
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1 file changed, 2 insertions(+), 1 deletion(-)
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diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
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index 3fbda1e3d4..9c46c3a8fa 100644
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--- a/hw/virtio/vhost-shadow-virtqueue.c
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+++ b/hw/virtio/vhost-shadow-virtqueue.c
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@@ -327,11 +327,12 @@ static void vhost_handle_guest_kick_notifier(EventNotifier *n)
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static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
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{
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+ uint16_t *used_idx = &svq->vring.used->idx;
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if (svq->last_used_idx != svq->shadow_used_idx) {
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return true;
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}
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- svq->shadow_used_idx = cpu_to_le16(svq->vring.used->idx);
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+ svq->shadow_used_idx = cpu_to_le16(*(volatile uint16_t *)used_idx);
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return svq->last_used_idx != svq->shadow_used_idx;
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}
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--
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2.31.1
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