From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:36292) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TipV9-0002ur-0U for qemu-devel@nongnu.org; Wed, 12 Dec 2012 11:49:48 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TipUz-0002FK-68 for qemu-devel@nongnu.org; Wed, 12 Dec 2012 11:49:42 -0500 Received: from mail-ie0-f173.google.com ([209.85.223.173]:33292) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TipJH-0007l3-Af for qemu-devel@nongnu.org; Wed, 12 Dec 2012 11:37:27 -0500 Received: by mail-ie0-f173.google.com with SMTP id e13so2178984iej.4 for ; Wed, 12 Dec 2012 08:37:26 -0800 (PST) Sender: Paolo Bonzini Message-ID: <50C8B2BB.50606@redhat.com> Date: Wed, 12 Dec 2012 17:37:15 +0100 From: Paolo Bonzini MIME-Version: 1.0 References: <1355322396-32026-1-git-send-email-pbonzini@redhat.com> <1355322396-32026-2-git-send-email-pbonzini@redhat.com> <20121212144448.GE15555@redhat.com> <50C8A0A8.3000303@redhat.com> <20121212153333.GC16750@redhat.com> In-Reply-To: <20121212153333.GC16750@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 1/2] virtio-pci: reset all qbuses too when writing to the status field List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, agraf@suse.de Il 12/12/2012 16:33, Michael S. Tsirkin ha scritto: >>> This is a device specific register, IMO it should reset >>> very specific things not what happens to be on the bus. >>> For example qdev resets the PCI header: will or >>> will not this reset it? >> >> qdev does not reset the PCI header. Only pci_device_reset (called when >> resetting the whole bus and also by FLR) does. > > Point is it's a simple register, the easier it is to understand > what happens on this write the better. The easiest way to understand what happens is to have a definition in the spec. There is none, so to me the simplest definition of resetting the device is "soft-reset the device", and in qdev a soft-reset is qdev_reset_all (see below). Whether it involves function pointers or not, I don't care. >>> Can't the required code just go into the virtio-scsi >>> reset callback? >> >> Of course yes, but it'd be different from all other SCSI adapters then. >> They don't expect that they need to do anything to reset devices on >> their SCSI bus. > > On virtio level, it's a device specific register, there's nothing > standard about it. If you are talking about some scsi thing here it > belongs in virtio scsi No, it's a generic thing. Other virtio devices may have a bus, and they should reset it just as well. virtio-serial's guest_reset function closes each port from its own reset callback manually (since commit 4399722, virtio-serial-bus: Unset guest_connected at reset and driver reset, 2012-04-24). That shouldn't even exist. The ports should close themselves when a reset signal reaches them, and the propagation of the reset should happen automatically just like IMO it should for virtio-scsi. Let's not hide the basic concepts behind "it's a SCSI thing". The concept here is that of a soft and hard reset, and these is the definition: - you soft-reset a device by writing to a register which is in the spec of the device (e.g., write zero to the status register); - you hard-reset a device by writing to a register which is in the spec of the bus (e.g., FLR); - hard reset is a superset of soft reset; - soft-resetting device X will also do a hard reset of all devices below X. For example, a soft-reset of a PCI-to-PCI bridge does a hard-reset of all devices below (qdev_reset_all calls pcibus_reset calls pci_device_reset). In QEMU, qdev_reset_all is a soft reset of a device. qbus_reset_all_fn is a hard reset of all devices below a particular device. There is no generic qdev function for a hard reset of a particular device (pci_device_reset is it for the PCI case, just to complete the nomenclature with something you're familiar with). These are all generic concepts. Nothing virtio-specific, nothing SCSI-specific. It's not open for questioning. :) In the virtio case you have (logically, not at the qdev level): virtio-pci | virtio-scsi / | \ disk disk disk Writing zero to the status register does a soft reset of the virtio-pci device. This includes a hard reset of the virtio-scsi device, and a hard reset of the disks. That can be easily modeled by qdev_reset_all(pci_dev). What we actually have at the qdev level is: virtio-pci ---> virtio-scsi (via ->vdev pointer) / | \ disk disk disk The soft reset of the virtio-scsi device is done by virtio_reset instead of walking the qdev bus, but even in this case the soft reset is perfectly described by qdev_reset_all. Hence, writing zero to the status register should use qdev_reset_all. > but apparently same applies to virtio-blk which > really has no block bus. virtio-blk has no bus, so it does the reset from its own virtio_reset callback. virtio-scsi needs to ask the SCSIDevices to reset themselves. Whatever virtio-blk does in its reset callback, the SCSIDevices will do---only in a "regular" qdev reset rather than a special-purpose reset callback. I can of course iterate on all the devices, but it is pointless when there is already a perfectly good callback to do a soft reset, and it's called qdev_reset_all. Paolo