From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54509) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YqcBp-00011q-BQ for qemu-devel@nongnu.org; Fri, 08 May 2015 02:55:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YqcBh-0003cs-28 for qemu-devel@nongnu.org; Fri, 08 May 2015 02:55:13 -0400 Received: from mailout2.w1.samsung.com ([210.118.77.12]:55030) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YqcBg-0003ZV-Su for qemu-devel@nongnu.org; Fri, 08 May 2015 02:55:09 -0400 Received: from eucpsbgm2.samsung.com (unknown [203.254.199.245]) by mailout2.w1.samsung.com (Oracle Communications Messaging Server 7.0.5.31.0 64bit (built May 5 2014)) with ESMTP id <0NO000H01QR1BW40@mailout2.w1.samsung.com> for qemu-devel@nongnu.org; Fri, 08 May 2015 07:45:01 +0100 (BST) From: Pavel Fedin References: <016001d088b3$8da9bfa0$a8fd3ee0$@samsung.com> <20150507153942.29afef5f.cornelia.huck@de.ibm.com> In-reply-to: <20150507153942.29afef5f.cornelia.huck@de.ibm.com> Date: Fri, 08 May 2015 09:45:00 +0300 Message-id: <00aa01d0895a$81d423d0$857c6b70$@samsung.com> MIME-version: 1.0 Content-type: text/plain; charset=us-ascii Content-transfer-encoding: 7bit Content-language: ru Subject: Re: [Qemu-devel] [PATCH v2 2/3] virtio-mmio: introduce set_guest_notifiers List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: 'Cornelia Huck' Cc: qemu-devel@nongnu.org Hello! > Hm, weren't there some patches for irqfd on arm? Yes, there were. However, they had a design problem by breaking backwards compatibility with unmodified virtio. Their idea was to set up one more shared memory area between virtio and vhost-net and use it to pass ISR value, which helps to distinguish, which event took place (queue update or config change). So, this idea was rejected. http://lists.gnu.org/archive/html/qemu-devel/2014-10/msg03056.html I thought about it, and technically, i think, this can be done in better way. Actually, as far as i understood, all we need is mechanism for distinguishing between these two events. On PCI we do this by using multiple IRQs via MSI-X, and one IRQ signals exactly one type of event. MSI-X code also has "two IRQs" mode as a failsafe, where one IRQ signals config change and another IRQ signals queues update (and all queues are polled in turn). I think a similar thing could be done for virtio-mmio. It could allocate two IRQs instead of one and describe both of them in the device tree. Guest side, upon seeing that, could make use of those two IRQs and acknowledge to the host side that "yes, i am new version and use new mode". But, sorry, i will unlikely implement this, because we already have PCI with MSI-X (i hope this is going to be published soon), so my project can use PCI emulation. So implementing irqfds for virtio-mmio is a bit out of my scope. Kind regards, Pavel Fedin Expert Engineer Samsung Electronics Research center Russia