From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 396B4C433F5 for ; Tue, 30 Nov 2021 14:14:44 +0000 (UTC) Received: from localhost ([::1]:39450 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ms3u7-0004bo-2J for qemu-devel@archiver.kernel.org; Tue, 30 Nov 2021 09:14:43 -0500 Received: from eggs.gnu.org ([209.51.188.92]:57596) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ms3Sg-0000yN-C3 for qemu-devel@nongnu.org; Tue, 30 Nov 2021 08:46:23 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:25189) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ms3SN-0003OG-UU for qemu-devel@nongnu.org; Tue, 30 Nov 2021 08:46:19 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1638279963; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=NNMhrC17Nc4UiLsjmwAwkFTZ+u+tqHjg0AYCS9rxQQs=; b=Cp372UxtX9y+VJaF7hgeXU18dnnATl+xy3yRICIoXYH7XRvr34iWtbVrYkBF34eV9ISnfJ O/KAf+vKbolv28PezOn03CVB8BG0JjxUEqQMe4bCEFkqoo/YclIHZ+HKFtV7ozgFtTjNI9 nfGxjZgHak0EKbJyAL7MEaz+95HiD+Q= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-52-Q1tfJCT_OtGghvl6daiXag-1; Tue, 30 Nov 2021 08:46:01 -0500 X-MC-Unique: Q1tfJCT_OtGghvl6daiXag-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C73D61B2C993; Tue, 30 Nov 2021 13:46:00 +0000 (UTC) Received: from localhost (unknown [10.39.195.31]) by smtp.corp.redhat.com (Postfix) with ESMTP id D252210016FE; Tue, 30 Nov 2021 13:45:11 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [PATCH] virtio: signal after wrapping packed used_idx Date: Tue, 30 Nov 2021 13:45:10 +0000 Message-Id: <20211130134510.267382-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="US-ASCII" Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.716, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, T_SPF_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jason Wang , Tiwei Bie , Stefan Hajnoczi , "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Packed Virtqueues wrap used_idx instead of letting it run freely like Split Virtqueues do. If the used ring wraps more than once there is no way to compare vq->signalled_used and vq->used_idx in virtio_packed_should_notify() since they are modulo vq->vring.num. This causes the device to stop sending used buffer notifications when when virtio_packed_should_notify() is called less than once each time around the used ring. It is possible to trigger this with virtio-blk's dataplane notify_guest_bh() irq coalescing optimization. The call to virtio_notify_irqfd() (and virtio_packed_should_notify()) is deferred to a BH. If the guest driver is polling it can complete and submit more requests before the BH executes, causing the used ring to wrap more than once. The result is that the virtio-blk device ceases to raise interrupts and I/O hangs. Cc: Tiwei Bie Cc: Jason Wang Cc: Michael S. Tsirkin Signed-off-by: Stefan Hajnoczi --- Smarter solutions welcome, but I think notifying once per vq->vring.num is acceptable. --- hw/virtio/virtio.c | 1 + 1 file changed, 1 insertion(+) diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index ea7c079fb0..f7851c2750 100644 --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -885,6 +885,7 @@ static void virtqueue_packed_flush(VirtQueue *vq, unsigned int count) if (vq->used_idx >= vq->vring.num) { vq->used_idx -= vq->vring.num; vq->used_wrap_counter ^= 1; + vq->signalled_used_valid = false; } } -- 2.33.1