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From:   Fei Li <lifei.shirley@bytedance.com>
To:     linux-kernel@vger.kernel.org
Cc:     Jason Wang <jasowang@redhat.com>, Pawel Moll <pawel.moll@arm.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Fam Zheng <zhengfeiran@bytedance.com>
Subject: [PATCH 1/2] virtio-mmio: Process vrings more proactively
Date:   Fri, 19 Jul 2019 21:31:34 +0800
Message-Id: <20190719133135.32418-2-lifei.shirley@bytedance.com>
In-Reply-To: <20190719133135.32418-1-lifei.shirley@bytedance.com>
References: <20190719133135.32418-1-lifei.shirley@bytedance.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Fam Zheng <zhengfeiran@bytedance.com>

This allows the backend to _not_ trap mmio read of the status register
after injecting IRQ in the data path, which can improve the performance
significantly by avoiding a vmexit for each interrupt.

More importantly it also makes it possible for Firecracker to hook up
virtio-mmio with vhost-net, in which case there isn't a way to implement
proper status register handling.

For a complete backend that does set either INT_CONFIG bit or INT_VRING
bit upon generating irq, what happens hasn't changed.

Signed-off-by: Fam Zheng <zhengfeiran@bytedance.com>
---
 drivers/virtio/virtio_mmio.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index e09edb5c5e06..9b42502b2204 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -295,9 +295,7 @@ static irqreturn_t vm_interrupt(int irq, void *opaque)
 	if (unlikely(status & VIRTIO_MMIO_INT_CONFIG)) {
 		virtio_config_changed(&vm_dev->vdev);
 		ret = IRQ_HANDLED;
-	}
-
-	if (likely(status & VIRTIO_MMIO_INT_VRING)) {
+	} else {
 		spin_lock_irqsave(&vm_dev->lock, flags);
 		list_for_each_entry(info, &vm_dev->virtqueues, node)
 			ret |= vring_interrupt(irq, info->vq);
-- 
2.11.0